just because it hasn't appeared in a previous Nintendo game doesn't mean they can't innovate new ideas into the game. My idea for the fireball merely comes from Miis not having any recallable traits.
Captain Falcon and Starfox pre-smash bros were characters confined to vehicles.
the fireball thing is more of a parody of other fighting games, as alot of games contain pretty much the same characters with palette swaps.
Ryu & Ken
Sub-zero & Scorpion
Mario , Luigi, and Dr. Mario
alot of Nintendo Wifi games have featured paint style customization
mario kart and animal crossing to name a few. So its not too far fetched.
The Miis are a culmination of years of Miyamotos research with Stage Debut, which arguably was in even better form on 64DD. Which allowed all sorts of customization.