Here are some rather personal suggestions and wishes, for future games as well as for existing ones, and maybe some of them are shared by other players.
1. Game parameter
For many things I'm going to talk about there probably is a game parameter like in CK2 needed, because presumably some 'hardcore' players wouldn't like them.
2. Governmental positions
What I'm missing a bit in AGEod games which are very detailed otherwise, are important civil positions and personalities like military and civil governors, eventually (lord) mayors, and especially ministers. In history, ministers were very important. Just look at Metternich, Schwarzenberg, Beust, Bismarck etc. And here's the first thing which hardcore fans of the existing AGEod games probably won't like: If ministries would be added, they can't be historical. Maybe they could, if the developers made it like that, but as I think it would make a lot more fun (and sense), if ministers could be exchanged. For example if a country lost a war and the ruler isn't content with his country's war minister anymore, what would he do? Of course, he would dismiss him. That's also one point which should be selectable at the parameter: Historical ministries/Custom ministries/No ministries.
3. Military leaders
I love the AGEod system of military leaders, really! But I also think that it could be improved in some ways to make it more enjoyable.
First: Assigning family members as military leaders (generals or admirals). As you all probably know, being a relative to the ruler was very helpful to become an influential military leader. And not all of these 'military parvenues' where untalented. Here, my examples are Archduke Charles, Archduke John, Archduke Albrecht (Austria), Prince Frederick Charles, Prince Frederick William (Prussia), Prince William of Oranje (Netherlands), Napoleon's brothers, and many more. Probably many of these leaders are included in the different AGEod games, but that's not what I'm asking for: what I'd like to see here would be a royal family, where the player would be able to select new generals, even if they weren't leaders historically. (Another option for the parameter: Royal family where you can choose leaders from/Royal family where you can't choose leaders from/No royal family). But that is not the main point and I could live without such a system very well.
Second: Regiment officers which can be promoted to generals. As I said before, I really like the military leader system in AGEod games which has much more 'charm' than Paradox one's. But what's a real pity in my opinion: there can't be assigned any new and unknown generals, only the historical ones can lead the army. Here I'd suggest a system like it's already used for some units in some games, it should just be expanded to all units. An example: in BoA II: WiA theres a unit called 'Marblehead', which contains a leader (I think he's called Colonel Seth Warner), and a unit of infantry. This system should be transferred to all untis, while these Colonels, Majors, and maybe lower rank officers like Captains for Artillery and Supply units, should be promotable to generals (only Brigadier Generals at first). The unit whose officer was promoted should get a knew one. (Parameter option: Promotable unit leaders/Non-promotable unit leaders)
Third: Possibility to assign military leaders civil positions. For example, a rather medium general who served well should be able to be assigned as the governor of a region which he conquered, military governor while the war goes on, and civil governor when it has ended. It should also be possible to make these generals ministers, mainly ministers of war/marine of course. (Assigning of military leaders as civil officials allowed/Not allowed)
Fourth: Assigning and removing staff officers. In Empire TW and Napoleon TW each general has a 'staff', but that staff has actually nothing to do with real historical staffs, because it can only be used for melee fighting, while in reality a staff never fighted. But Total War isn't AGEod, and so a realistic staff system could probably be implemented. It would be pretty similar to the unit commander system, like when you click on/hover over a unit, all staff members (smaller army/lower ranked army leader = smaller staff + lower staff officers, bigger army/higher ranked army leader = bigger staff + higher officers, no staff for armies up to three units, or armies which only consist of supply units) are shown. It should also be possible to remove these staff officers from the staff and make them leader of an army (or governor, minister etc.). The highest ranked staff officer should become chief of staff who has some influence on his commander. (Parameter option: Staffs for army leaders/No staffs for army leaders)
Fifth: Sending leaders as military attachés or advisors to other countries. This feature is quite dispensable, but it would still be cool if something like that would be added to a game. (Sending military advisors allowed/Not allowed)
4. More variety in decisions
Well, I think that the card decisions system is a significant part of AGEod games, although I don't really like it. But I think some custom and not historical ones could be included too, to allow more 'freedom of movement'.
So that's a small collection of my weightiest thoughts for any AGEod game. I hope someone reads this and gives me some feedback if anything of these ideas could be realised, although I surmise that most people won't like them.