I can't see all of the Disney worlds getting connected to each other geographically *ever* in the series, though it'd be cool to have Traverse Town, Twilight Down, Daybreak Town, Radiant Garden, the Mysterious Tower, Land of Departure, Destiny Islands, and the Keyblade Graveyard merged together into one giant-ass home world you can roam around. But merging all of the Disney worlds would probably lead to complications with the already complex Kingdom Hearts copyright lawyering, make it harder to introduce new worlds into the series in the future, et cetera et cetera.
But yeah, I wouldn't be surprised to see some sort of big shift in the status quo. I don't think Xehanort will die per se, by the way, even if his story is concluded. Death is basically against the rules in Kingdom Hearts, as far as I can tell. But I think we'll finally, actually get to see the real Kingdom Hearts in KH3 and that'll obviously be a very big deal (with the potential for basically a universe reset, given what the mythology has told us about it).