If I were you, I’d try to replicate the style of the original Smash Bros for the N64: with fewer polygons the characters would be easier to design and animate.
Or, since Smash is fundamentally a 2D game, you can follow the path of Rivals of Aether and go with the pixel art style.
Or mix the two styles, like for example low poly 3D stages with pixel sprite characters: if done the right way it would be really cool IMHO.
If you need help you should try contact the developers of Smash Remix, I can testify that they have a very friendly community