I also like the idea od changing environments. Please observe Ys series which, starting from Napishtim, became a series of 3D slashers similar to DMC or the two PS2 Castlevanias. In Ys Origin, when you fight the true final boss, Darm, your adversary can directly interfere with the arena, breaking it into pieces that change their positioning while Darm is still attacking. Quite the challenge. If the player falls into the pit, they are captured by Darm's tentacles and that costs them quite the part of their lifebar.