So, with the new AoE3, and considering that AoE4 is already confirmed to be settled in the middle ages (not in more modern era, like the napoleonic or the word wars era) I think that there are possibilities for a lot of new context and scenarios for the game.
The introduction of the swedes get my hopes up for the 30 years war, with the central Europe as the new maps/scenario and the various pieces of the HRE as the new tribes, there could also be new civs, like danish, polands, hungary and such…
The colonization of Africa also is a big possibility, since almost all mayor European power had colonies in Africa, and it open up the possibility for new maps, tribes, and civs, both African ones (like etiopians) and European (like Italians).
The napoleonic wars (I mean, napoleon represents the French, but other than that there is anything about it in the game).
The war of unification of Germany and Italy in the 1800.
In general, any conflict, war, conquest or colonization before the word war could be implemented through the main game, DLC and mods, and that means a lot of possibilities.