When it comes to Miis, I feel that they weren't handled the best. Not being allowed during online, multiple classes, etc. Personally, I hope we keep Miis, but just establish a default for them, that way they can at least be used in For Glory.
Establishing a default for custom characters is the exact opposite of what defines a custom character. What you propose below makes it much harder for a default to exist too, since the character's playstyle is too free form to balance and restrict to the point where most players can enjoy.
I do agree that Mii Fighters are poorly implemented because of the restriction of online use, but that is something the developers chose to do for reasons outside of their character design. Though I will admit the size-clause is indefinitely flawed, and as a Mii Fighter user, I hope that much will be removed and standardized so the Mii Fighters can receive proper balancing.
If it were up to me though, I would merge the Mii classes into a single Mii Fighter. Then, make the Mii Fighter the ultimate customizable fighter, with their entire moveset able to be changed. Move options would include the old classes' moves, new unique moves, or maybe even moves from existing fighters. Depending on the moves selected, the Mii would sport different gear. So, if the Mii has a move that uses a sword, than the Mii will have a sword on their back or hip. Same with other weapons, such as a gun, bow, etc. Lastly, allow for height adjustment and color selection straight in game, that way you don't have to back out and tweak with the actual Mii's height and weight.
That is a mess for balance and overall design of the character.
The class system is a clean way of allowing players to add someone they would love in Smash that follows a theme, which is the main reason they were added in the first place.
Wanted Zero? The Swordfighter exists.
Wanted Waluigi? You could make a Brawler.
Cave Story's Quote? He uses firearms, so Gunner is the place to go for him.
Your idea sounds really cool on paper, but going through the effort to add a character that can change any move between several options is just a mess.
Looking at the game competitively, people already complain the Mii Fighters have an unfair advantage for having the ability to have access to three options for their special moves that could use for counter-picking. Some areas allow them, other restrict to a specific moveset.
If a Mii Fighter had the ability to customize every attack option, they wouldn't be allowed to be used in competitive play. You have no idea what kind of character you're up against until you see what they can do in the match you're playing in. Plus, it wouldn't be worth the trouble to make sure they were using a different pummel or down tilt than they were allowed to. No doubt a Mii Fighter like that would be banned the day it was revealed.
The concept is neat, but it is too flawed to be realistic or enjoyable to play against.