I would honestly wanna see a playable Tails and/or Knuckles in the Sonic 3D Blast Genesis game.
However, I'd imagine that this would be a lot of work, since every animation frame has to be drawn from multiple angles. According to this video: https://www.youtube.com/watch?v=YV9x1KY_XWI
by one of the games' creators, Jon Burton , you need to have 9 angles per animation frame (some of which can be mirrored, resulting in a total of 16 angles). In this example a walking cycle consists of 108 individual frames (edit: if you count all different angles). Models of Knuckles and Tails do exist in this game, but each has only a couple of animation frames. So the rest of the frames would have to be drawn by hand or modeled and animated in 3d before converting them to 2d.
If you also wanted to add individual moves, like Tails' flying or Knuckles' punching and gliding that would also result in even more frames of animation and a good amount of additional programming/modding.
Now, I 'm not saying that it cannot be done (I'm in no way an expert on this), because there are some incredible hacks/mods out there! But it probably won't be easy.
But this got me thinking. How about:
Sonic Colors - 3D Blast (DX)
It would be different palette swaps for Sonic, similarly to how Super Sonic is done in the Director's Cut: https://www.youtube.com/watch?v=iFbki9Y-r_4
The following palettes would be cool: red (Knuckles palette), orange (Tails palette), green (Vector the Crocodile palette), black (Shadow palette), white/silver (Silver palette), purple (Espio the Chameleon palette), pink (Amy Rose palette)
The next step could be, if possible, refining some of these to make them actually look like other Hedgehog characters:
- Shadow: black, with some added white (chest) and red highlights (skin/fur), a bit of yellow around the ankles and wrists and white shoes with some red
- Metal Sonic: change eyes to black and red, skin tones and legs to metallic grey, and some yellow here and there
- Silver ...