I love the idea but I also would like to have time travel.. at least in a general sense of "if I kill my grandfather what will happen" be out of it as much as possible. For example, imo, using Echo you might get away from that- a player who has progressed say Ishgard restoration may activate their echo memories to see previous stages (but they did not travel in time per say, and if they did any quests they missed or whatever it would be suggested that you had always done those quests and were just remember doing them- there was no altering of the timeline).
Idea bred a bit from having fun with the idea we were a primal's primal lol but why our time is all messy (basically there is a time bubble), I ran with the idea that there is not simply one of us (you) and that you appear in multiple places (and times) at once but as a player (in real life) you witness these events linearly (but to your character is a jumble). We don't age, we simply exist through time at multiple points, but we don't "travel" in time like you might think of in Chrono Trigger either. They all happen "now" for you but each character you interact with time flows normally. (Also I'm not suggesting the primal nonsense is absolutely a thing / needed, it's just fun to go conspiracy / wild wacky theories sometimes as a "what if / could be" game - my point on relying on echo stands though).
Anyway I just really want to avoid "why not go back in time and strangle Zenos with a phenoix down" sort of stuff. Or like if we're winning why doesn't X bad guy do that to us. Etc. Etc. until suddenly it's a massive mess of why nots.
So anyway, would love to see zones continue to make huge shifts (and even larger) and I feel utilizing echo and other concepts to allow players to visit phases would be nice (like I feel Ishgard restoration maximal progress should be driven by the community but the actual current state should be driven by each specific player, echo then would help distinguish different states and how you might interact with them based on circumstance). Just hopefully be very careful with Chrono Trigger like 'time travel'. (I mean I still liked that game but for me time travel is something to accept from the onset, or to be done AMAZINGLY well, or just attempted to be avoided all together- like the Harry Potter time travel was just "......but y though?").
More on a content thought of it, I liked the FATE system added in ShB and thought it could turn into a more zone based system (married to some other concepts at the same time) to create a zone tier like master system. You'd have after X MSQ, side quest, and or beast tribe these evolutions to the zone that might add new paths, npcs, buildings, areas, etc. Naturally that'd use some phasing similar to beast tribes. Have to be careful to use so much phasing that you have to actually phase players of whole zones... (although given some sort of echo shifting concept you might get away with more, so long as the base 'state' of the zone is the one everyone shares for easy group play, hunts, and such).