Hey,
I have a bit of experience with Unity, and i'm pretty sure i could make a SmashBros-like game, but you'd need to be more specific about your vision.
Do you want the customization to just allow you to recolor a set number of textures/sprites or do you seek something more ambitious like procedurally generated characters and an extensive editor(think like the sims etc.)?
Either way, feel free to contact me, i'm always up for a conversation about your project.