Personal opinion; the next expansions should only increase lvl cap to 99, then 100
I haven't beaten the MSQ for Endwalker yet, but I'm assuming that most of us will be happy to see the game get another expansion, or even a FEW, depending if it can keep feeling fresh.
I thought about it a while back, but I feel like the next theoretical expansion should only take us to lvl 99, a very traditional "max level" in JRPG's.
Moreover, however, the expansion FOLLOWING THAT should simply increase the level-cap to 100.
I suggest these, because both are nice, clean numbers. Any time you go "beyond level 100", things just start to feel ridiculous. And realistically, I think most of us would generally be happy at lvl 99, but instead treating that LAST level to hit 100 as something truly special.
Hell, maybe that second expansion introduced "lvl 99.1, lvl 99.2" and so on, just to keep the same "feeling"?
But hitting lvl 100 should be something truly momentous, and realistically, perhaps be the end of your journey.
Or at least, if further progression is STILL required, perhaps look at other avenues at THAT point. For example, perhaps you can "ascend" your jobs, such as Dark Knight evolving into a "Chaos Knight", Warriors becoming a "Barbarian", Black Mages becoming "Wizards", etc? It would be hard to see progressing any further than that, but at THIS point, we're talking like 8-10 years in the future -- WHO KNOWS what that will even be like!
Point is:
I think lvl 99 feels like a really good "stopping point" for an expansion, and similarly, I feel like "Level 100" deserves something even MORE spectacular, which I think justifies it as being a level-cap for another expansion. Assuming each expansion lasts roughly 2 years, going to 99, then 100, buys us like 6 years before needing some "alternative". And at that point, we may be sick of "level-increases" anyways!