Well, here's my own take, in no particular order.
1. Benjamin Tennyson (Four Arms, Heatblast, XLR8)
2. The Powerpuff Girls
4. Johnny Bravo
5. Dexter
6. Chowder
7. Flapjack
8. Finn and Jake
9. Gumball and Darwin
10. Generator REX
11. Mordecai and Rigby
12. Samurai Jack
13. Steven Universe
14. K.O.
15. Captain Planet
16. Mac and Bloo
17. Uncle Grandpa
18. Courage
19. Ed, Edd and Eddy
And here are some guest fighters I'd say could work pretty well!
20. Fred Flintstone (licensed from Hanna-Barbera)
21. Robin, from Teen Titans Go! (licensed from DC)
22. Scooby-Doo and Shaggy (licensed from Hanna-Barbera)
And, well, here's some ideas of mine for DLC. I know these technically shouldn't be included, but why not!
DLC1. Optimus Prime, from Transformers (licensed from Hasbro)
DLC2. Kevin Levin, from Ben 10.
DLC3. The Bare Bears, from We Bare Bears.
DLC4. Space Ghost, from Space Ghost - Coast to Coast (possibly licensed from Hanna-Barbera?)
DLC5. Ice King, from Adventure Time.
DLC6. Tom and Jerry, from Tom and Jerry.
I know I sort of have a lack of villains, but as a sort of entry into the series I think it's a fair pick. Though there's a few guest fighters, I feel like they'd be fairly realistic in terms of being able to include them, with only really Optimus being a bit iffy for me. I also had to really restrict myself on the [adult swim] characters, so I felt that Space Ghost would work as a sort of rep for the whole thing, as well as fitting quite well stylistically with other characters here. As much as a teamup of the Aqua Teen Hunger Force would have been cool, Space Ghost seems to fit better in my opinion. I also considered characters like **** Dastardly, but he'd probably fit better in a sequel.