This is an idea I've pitched in a few threads (including Next Smash just now), but I don't think I've ever shared it in a profile post.
If I were in charge of Smash I'd probably greatly overhaul how Miis work.
I'd probably make it so a Mii's "class" is decided by a specific weapon choice that can be mixed/matched with headgear/tops which aren't fused with weapons. For example, a Sans Mii costume would have the mask, the hoodie with both hands free, and a Gaster Blaster weapon that gives the wielder Mii Gunner's moveset. This way, you could have a Swordfighter/Brawler/Mage Sans who looks like Sans, but wields the Master Sword or a Keyblade or something.
Not only would this be great for customization, but it'd also open the floor to add weapons that aren't tied to a specific character; Minecraft swords, a Zapper gun, etc.