We were discussing this in the AAR of Fable 14, but having some standards on what makes for a good third party role would probably be beneficial. My thoughts are below, feel free to add or modify them!
1. All factions should control their own fate. This is most often a problem with regards to third parties getting NKed. Unless they're something like a jester, where that's the Achilles' Heel, third parties who play very well should be able to achieve their VC, regardless of things outside their control. The same goes for other factions: An unlynchable third party who the town needs to get rid of is no fun, and neither is a bulletproof pro-town third party.
2. Third parties should be an actual THIRD party. Their victory condition should be directly at odds with everyone else's, and unless it's explicitly the point of the role to be on one team (i.e. the Aspirant in A Shot in the Dark), shouldn't be able to work with either side.
3. Third parties should have a reason to exist. Many games have third parties as the focus of the game - that's great. So is having a scary, sinister third party or an ominously named third party appearing over and over in flavour. They shouldn't be an afterthought or an attempt to make a game seem more fun.
4. Factions should all have approximately the same chance of winning with equal play. If the third party will win about 1/10 games, it's not good, and if the scum win 70% of games, it's not good. Aim for about 1/3 each, assuming reasonable play from everyone - this lets good play shine through!
Anything I missed?