In the past first games had simple battle facilities and then 3rd versions/sequels replaced them with a bigger one (BW added one without removing Subway)

SM already has two and none of them seems removable.
Battle Royal symbollises the 4 islands, 4 tapus and tradition so its very unlikely they'll get rid of it. On top of that its connected to Masked Royal so its even less likely they'll get rid of it
Battle Tree is a giant tree, standing out on the map. They wont remove it either.

So where would they put any new battle facility?
Try not suggest new islands because thats as likely as multiple regions