...assuming it is truly a new start, and we are going to a new area on our own.

I'm thinking, maybe branch off after Heavensward, and allow players to either continue into Stormblood or the new expansion. Have the new expansion not reference anything outside of ARR and HW directly, and it stays like that. If we do need StB/SB/EW links, do it in side quests or options, and wait until 7.4 or 7.5 before introducing MSQ that requires EW, if at all.

Mechanically, let the MSQ give you many levels if you use a 60ish character, and make 71 and 85 dungeons for the first two in the expansion. Give enough quests to maybe support a second job going to 90. Then continue as normal.

Xp scaling could be set so that 90 characters barely feel a bump, for those already at EW endgame.

It's awkward, but the MSQ length is getting unwieldy. Something like this should happen.