>>19289176For Stella I've always pictured a steel mill. I think from a visual standpoint it'd provide for some very memorable scenery with the dull grays contrasting the bright red/yellow/white of the molten steel.
As far as a puzzle goes, pic related is a very rough example.
When you first enter the gym, it's very dull/gray for the most part. You go around the gym turning valves / hitting switches that pour giant containers of molten steel into a path. Stella is located on a platform that is detached from the main path (blue X in the image), but the path that the molten steel fills runs between the main path and her platform.
After you've let loose the molten steel, you have to hit another switche / valve that raises two floodgates on both sides of the path (blue arrows). Then gates then close and the molten steel inside would harden, forming a walkable path to Stella.
Just an idea, not even sure if that'd be possible to implement. Though now I kind of want to make a proper sketch for it.