But for more modern variants the units allowed seem to strain credibility. Are modern war machines really limited to engaging enemies in adjacent territories? Is a war of conquest between nuclear powers really likely to be low-attrition as diplomacy models it? Probably not, and for many of us who don't mind a little bit of tactical variety those facts both strain suspension of disbelief, and represent lost opportunities for new and interesting interactions to be modeled in-game.
For this reason, several variants have been created that include either aircraft or nuclear missiles as units. All of these variants however, have fallen victim to one of two problems: either the introduction of units with realistic (or even quasi-realistic) abilities along these lines disrupt the effective flow of gameplay, or the rules necessary to model these units accurately and maintain an interesting game, are too complicated to keep players interested.
For instance:
In the case of aircraft, a first cut at them might be to create a wing unit that can move across land or sea. Aircraft can fly anywhere so this makes some sense. This however breaks the game as those wings are then strictly better than armies or fleets and there is no reason to build anything but them. Another rule can be added forbidding those wings from taking supply centers since planes cannot take territory. This also makes sense. Unfortunately it makes wings almost worthless since the ability to capture scs is most of what makes a unit valuable, and while all-terrain ability might be worth it in some cases they are few and far between. Additionally neither of these rules fixes the issue of planes being limited to the slow movement of diplomacy which is unrealistic. Wings in dissolution are a different variety entirely. They can't move (except to sally and bounce) but they can support and convoy and importantly are all considered adjacent to each other for convoy purposes. This, I think, is the best implementation of wings that I have ever seen or played, but it still suffers from two problems: first building a wing makes you vulnerable since the enemy can now use their own wings to airdrop onto the supply center in which you built the wing, and second, the wing can't bomb, or support, or do anything in any space other than one occupied by another wing (or adjacent to the first one) these are both unrealistic and together make building wings a bad decision that few ever make (some do in dissolution purely because there are a bunch of variant-specific indestructible neutral wings to convoy off of, but in any other variant use would be essentially zero). On variant-bank one can find a set of wing rules that relies on actions like "raid" "escort" and "patrol" which fix all the problems of wings and implement a satisfying addition to the game, but they are easily three times as complicated as all the other rules in the game of diplomacy combined.
In the case of nuclear missiles it's simple enough to have a zero-power army that can, as it's move, sacrifice itself to destroy any other territory on the map. This creates a serious problem though: nukes are too powerful. In the fall everyone launches all their nukes at players who lose scs and units and then have disbands so they can't rebuild and retaliate. Whoever did the best diplomacy before that first fall phase wins. Very short very unrealistic, very boring, game. This can be solved by pausing the game between phases to give players a chance to retaliate for nuclear launches, but then why not allow retaliation for non-nuclear moves? The simultaneous nature of diplomacy quickly breaks down. New World Order attempted to solve this problem by allowing their players to issue conditional orders to their missiles, but, while I never have seen how one of those games played out under that ruleset, I'm guessing that that solution is both a nightmare to adjudicate and an invitation to paradoxes.
This variant that I have created attempts to overcome these problems. The map is just one on which this ruleset can be used, but it has been designed with these rules in mind and so is probably a better map than most to use when playing with said rules. It is meant to represent the roughly modern world. Since modern borders do not conform to diplomacy notions of a good game some heavy editing had to be done with the result that the placement of both borders and supply centers do not always make sense. The map is set up the way it is for gameplay purposes, not because I think that Chile isn't rich enough to deserve to be an s.c. or that I think Israel shouldn't exist (given as examples, not as a comment on what I might or might not think).
So here are my proposals for how to make nukes and planes work in diplomacy:
Builds:
On any winter phase players may elect to build a missile battery or an air wing in place of an army or fleet. Wings and missiles contribute to total unit count in the same way that armies and fleets do (total wings+missiles+armies+fleets must equal total number of scs after adjustments). The number of builds or disbands available to a player is determined by the number of scs that player controls at the START of the winter phase in question. Given the existence of missiles that sc count may change during the winter phase.
Air Space:
On the map collections of territories are surrounded by thick black borders rather than the usual thin black borders. These borders govern the movement of armies and fleets in the same way that ordinary borders do. The significance of these borders is that all territories encompassed within a single thick border are under a common "airspace."
Wings:
Wings Actions:
Wings have three kinds of actions they can perform, just like fleets do. These orders are: move, support, and "bombing campaign." They may not convoy. They may not be convoyed.
Wings movement:
Wings may move from any space to the airspace above it, they may move from any airspace to any adjacent airspace, and they may move from any airspace to any friendly-colored territory beneath that airspace. A territory is a friendly color if either 1) it is colored with the color of the wing's owner at the start of the game, and is not a friendly color for any other player's wings or 2) if the territory were a supply center it would be under the control of the power controlling the wing (i.e. it was occupied in a fall turn by a unit of the player).
Wing Support: Wings may support actions in any space into which they could move just like all other units. This means that wings may always support actions in the airspace above them, in airspace adjacent to an airspace they occupy, or in friendly territories below an airspace that they occupy. They may not support actions in ground spaces adjacent to their own. They may not support actions in sea spaces unless those spaces have previously been occupied by a fleet belonging to the same power and no other power has since occupied it with a fleet in a fall phase. Planes moving to a sea space represents them landing on a carrier.
Bombing Campaign: When a wing is in an airspace that wing may, in lieu of taking other actions, be ordered to carry out a "bombing campaign." This consists of the wing not trying to land in a space suitable for it, or to fly somewhere else, but rather to patrol the area in which it currently resides looking for enemy targets to bomb. When a wing does this its controller's units win all battles that take place in spaces beneath the airspace of the wing that would otherwise be equal. This can be thought of as giving each of the units of the player beneath that airspace 0.01 additional power (never enough to prevail on its own but always enough to tip the balance of a tie).
Disambiguation:
When a player engages on both sides of a conflict, say by supporting two powers into the same space, the tiebreak provided by the bombing campaign goes to the side on which more units of the wing controlling power were participating (two supports beats one support). Convoys do not count towards this adjudication as they do not contribute directly to the force applied to the battle. This means that if the power controlling the wing conducting a bombing campaign supports a unit of one power into a space under the wing's airspace and convoys another power into that same space, the moves do not bounce even if otherwise equal because one has "more air support."
Nuclear Missiles:
Missile Battery Actions:
Missile batteries have three actions like fleets do. They may move, support or launch. They may not convoy.
Missile Movement:
A missile may move to any adjacent land space, or any space to which it is convoyed by a fleet. Missiles in other words move like armies do.
Missile Support: Missiles may support actions in any space to which they could move without a convoy. Missiles support like armies do (tactical nukes spotted from the launchers). Missiles move and hold with a strength of zero and cannot capture supply centers if they occupy them at the end of a fall phase.
Missile Launch:
A missile may be launched at any territory, land or sea, within range. For the map above every territory is "within range" of every other territory. If different technological assumptions are desired in a given variant then range may be limited to territories under the same airspace, or under adjacent airspaces, or within a certain number of territories, or by any other criterion appropriate to the historical moment in which the variant is set. Missiles may be launched in the winter as well as spring and fall phases, but they may not move during the winter phase. When a missile is launched at a territory then, before any other moves are adjudicated, all supply centers and units in that territory are destroyed and the missile battery itself is consumed and removed from the map. This is the only way a missile can affect supply center count. A missile ending its turn in an uncontrolled supply center does not change its ownership.
Disambiguation:
A missile that strikes an sc in a winter phase destroys the sc before any build in that sc took place. Since a build requires an sc in which to occur the build then fails. If a missile strikes a unit moving or supporting that order is canceled (it required a unit to carry out. Nuclear cinders can't bounce or support.) A missile launched in the winter phase does not count towards units on the board for the purpose of computing builds for the player launching it. So you can launch a missile in place of a disband, or to get a build, but you can't build in a supply center occupied by a missile even if you launch it because the sc has to be clear at the start of the winter phase.
Rationale: The challenge is to make a missile and wing which are neither overpowered, nor underpowered, realistic, and simple, in line with the generally simple rules of diplomacy. Missiles and wings have the exact same move and support rules as ordinary diplomacy units have: namely that they can support into any space to which they could move. Missile movement does not differ from army movement at all, so this is kept simple as well. Missile launch is a departure from standard diplomacy, but it is done in the most simple way possible: pick a space and everything in it dies. There is a little bit of added complexity by allowing missiles to launch in the winter, but since the move is exactly the same and doesn't affect the computation of builds for anyone but the launcher if he is disbanding, this should add minimal complexity. This feature of missiles is needed to prevent them from becoming overpowered. Simply a player hit with a missile must be able to retaliate before disbanding. Letting him launch the missile on the phase on which he would need to remove it enables this. Delaying the affect of missile strikes on unit count until the next winter phase if they're launched in the winter prevents the circumvention of this affect by the first launcher waiting until the winter phase to fire. In the case of wings the movement system is indeed new, but I think they add no more complexity to the game than did fleets. Air units have a new kind of space on the map that only they can enter: air space. Fleets have sea spaces which only they can enter. Air units have a new kind of movement restriction: only to correctly colored (controlled) territories. Fleets also had a new kind of movement restriction: accessible along the coast. Air units have a new kind of action: bombing campaign. Fleets introduced the convoy action. The rules for bombing campaigns require a bit of disambiguation. The rules for convoys do as well (and can create paradoxes unlike the rules for bombing campaigns). Because all of these wing actions interact through the usual diplomacy mechanisms of movement and support, I think that they do not add overmuch to the complexity of the rules.
Alright, that's all except for the variant map. Anyone who wants to sign up to play it with the new units PM me. Map notes: Middle east has a northern and a southern coast, Arabia, Quebec, Toronto, the Philippines, Indonesia, Scandinavia and Great Britain all include some connected water space. Fleets can't convoy through these. Territories marked in darker brown (not the color of India) are impassable. Any island or body of water not named cannot be moved into for game purposes. North West Pacific borders North-East Pacific and mid Pacific around the world and South West Pacific borders Mid Pacific and South East Pacific the same way. Some of the areas are named in ways that are not geographically correct. I will probably update them before any game. Lastly 40 sc on map, 21 needed for a solo.

https://imgur.com/frc0yKj