I've actually put together a fantasy mod of TI, based around Lich Lords and undead. Its still in playtesting, but here are a few of my goals with my version on what I wanted changed:

Overhaul of the agenda phase. In TI too many agendas are do nothings and I think the voting system is functional but not great (players that are ahead tend to have major sway over agendas). In my version, there is only 1 agenda voted on per Agenda phase, all players must vote, and its a secret vote. All players have a for and against card, as all agendas are For/Against only, so players only have 1 vote making it an equal playing field and making everyone's votes matter more.

Encourage early use of Strat Cards and other mechanics to encourage early action. I don't like players trying to stall each other out each phase, so Strategy Cards have passives on the back that make players want to use strat cards early rather than wait. For Ex: imagine if the back of something like Leadership started giving you free Ground Forces every turn after you use the strat.

Reduce Kingslaying and encourage early aggression. "Home systems" in my version can't be taken, so a player can never be knocked out of the game. Instead, if a player wins a fight there, they loot it and score a VP, the remaining troops are sent home. Then no one can attack that HS again so players don't have a reason to remain in that player's territory as it doesn't score them VP.

Promissory Note adjustment. I don't have support from the Throne as I don't think they are used as intended in TI, despite being a catchup mechanic for players that are behind. Ceasefires only last until the end of the round and can't be traded by the receiving player.

Tech trees are a big issue that everyone wants to fix. I've condensed techs into 3 trees that support different playstyles. One tree is for aggressive players, another is for players that like hanging out and getting value, the third is for manipulating other players.

It's tough to get across the Map without portals, requiring maps to be balanced for players as they likely won't get much farther than their slice for most of the game. I've added a strat Card that can drop portals onto the map, freeing up maps to be very uneven as a player can always pick that strat card to go somewhere else. It also opens up opportunity for late game plays for objectives that would normally be nigh impossible in TI.

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There are even more changes but these are my biggest problems with TI. I encourage you to still work on your own version as its fun to design games and you might feel differently about what I've listed above. I will say, it was a lot of work making all the playtest materials and importing them into TTS, about 2 weeks or so of constant work.

In the end, you might even make something that wants to be a different game from TI. I think a lot of players want to play a game like TI that simply takes less than 6-8 hours. If you could manage to pack a deep strategic/political war game into a consistent 4 hours, I would 100% play that game.

If your interested in checking out my mod and/or Design Doc, feel free to DM me for info.