It's always possible to give Mach Rider little fire boosters on the bottom of his bike, the way Samus actually jumps via a small jetpack.
Honestly all I want for DLC is the return of cut veterans. I mean, I do want other characters to show up, but I don't care about newcomers as much as I do the return of elements from previous iterations. I'm talking every character by the way, even if they have to be reworked as new characters:
Roy: Marth was changed a lot in Brawl and more in SSB4, to a point where I think he and Lucina would play distinctively from Roy if Roy were to return unchanged. Naturally, he'd have an Eliwood alt.
Pichu: Pichu should have been in the game originally considering it's one of the non-intensive "duplicate" characters Sakurai was talking about...
Young Link: Controversial here, I know. Young Link's style was taken by Toon Link, but that doesn't mean he has nothing to offer. I'd give him a Majora's Mask-enfused moveset with Bombchus, Deku Nuts, Deku Sticks, Great Fairy Swords, and of course... Masks! He could also take advantage of unrepresented franchise staples, like candles, magic staves, etc. He would have alternate characters representing the other children Link from Zelda lore: The Legend of Zelda Link and A Link to the Past Link.
Squirtle, Lucas, Ivysaur, Snake, Ice Climbers, and Wolf don't need any changes other than balancing and proper Down B and Final Smash for the Pokemon (for Ivysaur, he could definitely use Solar Beam as a spiritual successor to Triple Finish).
I'd also want some alternate characters for the Zelda cast...
Link: Ocarina of Time Link, Skyward Sword Link, Hyrule Warriors Link, Zelda II Link (voiced by Jonathan Potts), Ocarina of Time Dark Link (actually announced "Dark Link" and with dark versions of Link's tools)
Zelda: Ocarina of Time Zelda, Skyward Sword Zelda, Hyrule Warriors Zelda, Cartoon Zelda (voiced by Cyndy Preston)
Sheik: Ocarina of Time Sheik, Super Smash Bros. Brawl Sheik, Hyrule Warriors Sheik
Ganondorf: Ocarina of Time Ganondorf, Aghanim, Wind Waker Ganondorf, Demise, Hyrule Warriors Ganondorf
Toon Link: Conductor Link
Reason: Each of these "costumes" or "alts" are technically different characters, some of them actually veterans. I'd like to see them appear in at least costume capacity.
For newcomers, I couldn't really care less who they give us as long as we get most of the veterans back... I guess here's a top five anyway:
1: Tetra (Toon Zelda would either be an alt or absent entirely)
2: Ganon (Blue Pig Ganon design with trident as primary weapon. Uses the cartoon incarnation of Ganon as an alt)
3: Lolo (Has a Lala alternate)
4: Captain Toad (Has alternates for all members of the Toad Brigade in both Captain and traditional attire. Same for Toadette)
5: Dixie Kong