Maybe they will simply make a portal once you finish off the plains boss, that you enter and find yourself at random location on map with new boss stones.

Since you still have mistlands, firelands and deep north bosses. And once you finish those, maybe a new portal spawns and that one ends on top of map (like that large tree you see in sky that leads to final boss).

Kinda like a Sky area from terraria for example:
https://terraria.fandom.com/wiki/Floating_Island

Final boss on Yggdrasill branches would make sense:
https://en.wikipedia.org/wiki/N%C3%AD%C3%B0h%C3%B6ggr