My idea for a possible Crash Bandicoot stage is the Warproom from Crash Warped. Not only i choosed it for the mechanics that i will explain, but also because of the fact that Warped is the most popular game of the franchise in Japan.
The mechanic would work similar to Kalos League, but with some differences: you start on the place itself, but you will also get warped during the fight to the boss stages from the game. On each one, you will find most of the time the enemies found on the stages based in the look of each boss section in the Warproom (the Colliseum where you fight Tiny would include enemies from the Medieval levels, N Gin’s mech the ones from the Egypt ones, etc...). But sometimes, you will find instead the boss found on each one acting as hazards.
The cool thing is that the layout of the stage changes everytime it warps you to each section, so it would be a bit more complex compared to Kalos League on that regard.
Being honest too, i wouldn’t mind the idea of the stage reussing the assets from the N Sane Trilogy: the artstyle fits enough for Ultimate.