Mario Kart Roster?
Well, I would make sure every Smash character is in there, as well as the Bosses(Master Hand, Crazy Hand, Giant DK, Metal Mario, Giga Bowser, etc.), as well as include some other transformations like Super Sonic and Wario Man. I feel they all at this point deserve their spot. Not just because they're in Smash, but they're all awesome options and none should be ignored. Beyond that, Dixie Kong too.
Hard to say on what base should be, but I got a bit of an idea;
Then the various Bosses, other Alts not shown here, and all the Unlockable characters in general from the Smash games not listed here. And Dixie Kong.
The DLC otherwise is going to be the 3rd Parties overall. Others I think would make excellent DLC, due to selling well and gathering attention is Isabelle, Ridley, King K. Rool, and Piranha Plant. Maybe Metal Mario could fit well in here too. Any other DLC characters we still have to come would fit here in this case(if 1st party, maybe the Unlockable roster. It really really depends who). Other thoughts are Isaac, Bandanna Waddle Dee, Shadow, Tingle, Impa, Midna, and many other really popular AT's/Spirits who aren't playable. Dixie is the main exception cause she was at one point intended to be in Smash, where most others aren't even confirmed for that much alone. If I were to push it, maybe a surprise appearance by James Bond and other characters considered who never made it in. But ehhhhhhhh, Dixie feels like enough. The roster is already huge for a racing game anyway.
I'm unsure the best way to do Pokemon Trainer here. Just the Trainer, or each one separately as well.