I am a long-time TGC player who hasn't played as much recently due to overplaying it and this is a huge project as the best gameplay with this game is to start in 1914 (for more role-play, building up minor/regional powers and to avoid late game lag). I see this as fantastic and I don't know if it will be event-driven or decision-driven but a few thoughts for food:
-For long-term play, leaders should remain and never retire and there should be more for each of land, naval and airforce to allow expansion.
-Gameplay lag of energy/resources shortage needs to be fixed and I see 2 ways... 1 is to replicate the energy dilemma of Japan and then have more resources in the "post-WW2 era" or simply keep the gameplay resource focused as expansionist powers seek to control areas with necessary resources (with resources changing as an option for extra simulation).
-Diplomacy and intelligence is dead and that's fine. Let's focus on the main gameplay issues. For more long-term gaming, I would love to see, for example, South American nations uniting in a defensive alliance the moment 1 gets attacked (ie. so playing Brazil, Argentina, or even Portugal or Spain to renconquer South America triggers "an all out war" to make it harder, rather than isolating each nation and attacking it).
-Main issues is AI building armies, forming navies and amphibious assaults. This is a big task and I defer to the modders but goes to the core of a long-term playability game.
-Toggling nukes on/off should be a must for players like me and many others who do not like to use nukes due to AI limitation.
Good luck, I am hugely excited and if I can help in any non-modding way, please do let me know (as I play a lot of this game and have a genuine interest in a long-term playable mod)!!!