Copy-pasted from a thread I (accidentally) made.
I believe they will add a whole new level to the meta-game if it meets the following criteria:
1. Not all moves are changeable.
2. The properties of the move change
3. The individual percent properties are not customizable
What that means is that when you choose one of the "new" moves, it would tell you how the moves is different, but not let you specifically change things such as speed, knock-back, damage, ect. Because in fighting game, all your moves are just tools, and a tool has a specific purpose. In a game, you're essentially using your tools to try and beat your opponent and their tools. But perhaps you don't have the right tool, or find one of your tools lacking for some reason. So of course you'd want to switch tools, and that's exactly what I hope custom moves brings to smash 4.
I'll be using Mario's fireball for an example. In the direct, we saw 2 variants of the fireball, one that is slow, heavy, and hits multiple times. Then we saw one that's quick and has no knockback. So, lets say in some match-up, I want to be able to edge-guard better, so I choose the slow and heavy fireball, because I want to shoot it off the ledge when my enemy is trying to recover from below, and I like how the new fireball multi-hits and lingers. I can't really apply pressure with it but I felt it was a good trade-off. Or perhaps I prefer to zone my enemy and tack on damage before going for the sure-kill. Then, having a fast, accurate fireball sounds way better than having to ledge-guard.
However, if I change all my moves, am I really playing the same character? Also, if I could just make a giant, fast fireball that insta-kills... Well, that should be self-explanatory.
So, say I have you convinced this is how things will go, what would be an easy way to implement this? In what I believe to be a perfect world, tournament legal custom moves would look like this. First you pick your character as you normally would, then perhaps there would be a little icon you could select on your character's picture. Then there'd be a scrolling list of moves that appears over the character like if you were choosing a name and on that list there's all the customizable moves. Next, you'd choose the move you want to customize and it's give you 3 options, the normal move, variant 1, and variant 2. Once you were done customizing you'd hit "done" and you'd be ready to go.
So if I wanted to play Mario with a different fireball, then I'd simply choose him, hit the edit icon, go to fireball, and choose the fireball I want, while the other players choose their character and make whatever changes they want.