I have quite a big suggestion, but I'm sure many will like it.
In AOH3, features for diplomacy with other nations are very basic, even worse than AOH2 I dare to say. You cannot have more than one ally unless you max out the "Alliance expertise" legacy, you cannot punish your enemy's ally that fought against you in the war, and there is nothing stopping you from leaving your AI allies to fight all alone while they always help you in your wars, essentially becoming a tool of you, among other things.

My suggestion is an overhaul of alliances. Instead of alliances working almost exactly like a shorter defensive pact, I suggest having alliances work somewhat like factions in Hoi4, they can have an unlimited number of members, and no expiry date. When war is declared against one member of the alliance, every other member will be requested to help or suffer a major legacy and unrest loss as well as being kicked out of the alliance and receive a penalty to the likelihood of success score should they try to reenter the alliance in the near future.
When war is declared against one member of an alliance, the war will NOT END until all major powers and the alliance leader (nation that created the alliance) are defeated. Then, peace will be able to be made and the opposing side can demand concessions from every nation in the alliance that fought against them.
If the alliance wins against the enemy, each individual member can use their war score (that is obtained as if they were fighting a regular war) to demand concessions from the opposing side.

I hope I am not asking too much and you will consider my suggestion. Do not hesitate at all to ask questions if you are considering this and need clarification. I admire your efforts to improve this game with your mod. Thank you.

Suggestion 1) advisors in the game are not very necessary
the price of advisors should be increased and at the same time their features should be buffed
So will become really necessary

Suggestion 2) It should be cheaper to relocate the capital

Suggestion 3) no maintenance cost for many buildings; Buildings become an important factor if buff comes to many buildings and at the same time there is a huge amount of maintenance cost