Who remembers Marioguy0's Boss Shop
For those who don't remember, it looks like this.
The upper thing is more similar to the Normals team shop, and the lower thing looks way cooler from the exterior (and kinda cool on the inside, too) at the expense of having a much higher brick count. The idea here was to have the upper room contain power-up items such as "30 seconds of Chaos Armor" and have the lower room contain boss classes that are tweaked versions of existing ones, having benefits and drawbacks, but overall being a little bit more powerful. Maybe we could stick the old Mocheeze in there.