I think that “cohesion” in this case can be understood in two distinct ways: aesthetics and game play. If we’re talking aesthetics (as in the first suggestion), I agree completely! There’s been a lot of buzz on discussion board about how further aesthetic developments can be brought into the game. New unique castles for all the civilizations are a big one (and I think a must in the long-term). More regional skins for buildings and units are other options that could be a lot of fun! Personally, I’m in the “more-the-merrier” camp, though I sympathize with those who want to prioritize the readability of the game - that will be very important to preserve, too.
As for cohesion via unique gameplay, that’s a bit tougher. Adding new mechanics and features can (and indeed should, if its worth its salt!) transform the strategies, options, and viability of the affected civs. Balance is a very delicate beast in AoE2. Plus, something we can’t overlook (as we might be prone to as AoE fanatics) is that it’s important to have simple, straightforward “beginner” civs like the Britons and the Franks - so the devs would have to be careful about messing with the beginner-friendly nature of certain civs. And I haven’t even broached the subject of however important “legacy” should be in guiding their decisions. Is there such a thing as going too far from the 1990s roots?
Having said that, I for one am in favor of making these sorts of changes too! I came up with my own list of off the wall ideas here. But I’ve got to say that I think the FE devs are doing this perfectly by moving slowly and piecemeal through the old civs, rather than taking an “all at once” approach. It started with balance updates, of course. Then more attributes were introduced to civs, like the Teutons additional melee armor. Then regional units were introduced and added to the Lithuanians. And now we have a total rework and renaming in the Indians/Hindustanis. They’ve been slowly introducing these kinds of changes to ease people into the notion and garner feedback, and that feedback has been positive I think. I think it makes sense to continue spreading out these sorts of changes rather than take an all-at-once approach to, say, regional units or unique buildings. Either way, I’m really excited to see what they’ve got planned for the future of the game!