Cool idea, but I can't see it happening.
The only customization I can see happening is semi-creating a Mii character.
Each Mii has a default size, weight, speed, throws, properties and normal moves, but you can choose between a pack of special moves to apply to whatever Mii you choose. This pack includes the b moves and the final smash, but all balanced out that it just comes down to preference.
You register that moveset to the Mii you choose to play as (you can choose any Mii you want, and choose the special moveset you want)
Complete customization seems too much, but still somewhat realistic.