I think AoE2 is sort of getting stale; I think it’s a certainty that splitting the Chinese civilization will happen, simply because the Romans don’t really have a place in AOE2, yet they got added anyways as we’ve covered virtually every part of the world.
Central Asians? All sorts of obscure Central Asian empires got added. South Asians? Dynasties of India gave us a look at various interesting Indian kingdoms. Southeast Asians? Rise of the Rajas. Even the ###### ended up being featured via The Forgotten, and various African civilizations were added.
About the only place Forgotten Empires hasn’t touched yet would be the various cultures in the Sinosphere, and breaking up the Chinese is probably the only option remaining, which should get us 4 civs (Tang Song Ming Qing, the latter via the Jurchens), then add their sparring partners (Goturks to avoid saying Uighur, Khampas, to avoid saying Tibetan, Khitan are yet another steppe civilization that might have interesting gameplay.), This’ll cover two expansions.
However, how about revamping some core game mechanics? AOE ends roughly in 1600; the most interesting battles of the Jurchens occurred roughly around 1640. One opportunity to get new ideas might be to expand the scope of AOE2’s time period; we start as early as 450 with the Attila the Hun campaign, and end as late as around 1590, with the Battle of Noryang Point.
If we end currently around Noryang Point, we’re actually getting pretty advanced in time. If the Imperial Age represents the High Middle Ages, we’re already past the Italian Renaissance with Noryang Point and getting a 5th age might keep things fresh.
To get a 5th age, we could do it conventionally, i.e, make it possible to trivially upgrade from Imperial to Renaissance or Enlightenment, but the problem is that it highly disrupts the current meta, where Imperial Age warfare eventually leads to trash wars.
A more moderate solution is simply to have the Wonder of each civilization represent that it’s reached a Renaissance or Enlightenment for the respective civilization.
That is to say, we add in a 5th age, but have the 5th age be bound to the Wonder.
I.e, the wonder can research 5th age technologies, build unique 5th age units (arquebusiers, for instance, or rodeleros, light cannon, etc), or provide other bonuses.
In Standard Victory, this helps make Wonder victories easier to achieve, since the Wonder holder now has the ability to build units superior to those of their attackers, and helps make fights more decisive (since now the best counter to a Wonder is another Wonder, and it’s not simply a matter of destroynig the opponent’s wonder, but destroying the opponent’s wonder and keeping one’s own wonder alive).
In Conquest Victory, wonders are essentially useless decorations, and making Wonders grant Age 5 technologies and units helps make wonders a viable strategic choice; they are now sort of an Imperial Age Castle, without the natural attack capability.