They should just take a page from the .hack playbook, in those games all your stats/items carried over but the level cap was higher in the next part. People don't lose the stuff they worked on in part 1, but also can't overlevel for part 2, and equipment/materia should naturally be better in part 2 so that'd be no issue. That only leaves items, and if people want to grind out a bunch of items to make the next game easier, well, whatever