So, I like Sonic. I like Mario 3D world. What if the two came together... as a mod. Now before you say "Why not put it over Toad, he's the fastest!" It's because Luigi is (a) tall, (b) has the highest jump, and (c) he is second-fastest. If ui mods are a thing, go ahead and add extra life and Character Select icons. And if you wanna go the extra mile, add power-up forms. Here's what the power-ups should be: Fire: Red Sonic. Tanooki: Plaster Sonic's face over Luigi's. Cat: Same idea as tanooki but make him blue. I hope someone makes this a thing but I don't think many people know about the SM3DW molding scene enough to care. A model I suggest using would be the Sonic Unleashed model. It's fairly high poly so I think it would flow well with the game's aesthetic. I made a mockup but it uses the smash 4 model and the mockup kinda sucks.
Why replace Luigi when he's the best character in the game? If anything, Mario or Toad would be the ones to replace, since Sonic 1. Isn't known for jumping high 2. is fast 3. Wouldn't ever replace Luigi.