The Amazing World of Gumball - Hilarious cartoon with tons of great characters and source material to pull from. Get some of the show writers on board and it could have some of the funniest callouts in pinball.
Pokemon - I'm not even a Pokemon fan, but it's undeniable that it's huge and loved by people of all ages. The game's concepts of catching Pokemon and making teams to fight with could also lend itself to a unique ruleset. Imagine if each Pokemon had a small perk, and if you could evolve them to increase the effects of their perk. Another idea could be to have catchable Pokemon appear on the playfield via a hologram (Like Dialed-In) with a magnet below. Then, when catching Pokemon, you would shoot the ball at the hologram, the ball would be held below it by the magnet, and an animation making it look like the Pokemon is getting sucked into the pinball (Like a Pokeball) could play. Could be really cool. Plus, Pokemon Pinball has been done before on Game Boy and Game Boy Advance. It worked really well there, don't see why it couldn't be even better IRL.
Donkey Kong Country - This isn't really a no-brainer, but I'd love it. Get David Wise to do the music and we'd probably end up with one of the best pinball soundtracks in a while. The series' mechanics also lend itself well to pinball, particularly the barrels, which could be used as cannons.
Chicken Run - Again, not really a no-brainer, but I love the movie and there's some great potential for cool toys from the film. Plus there is a sequel in the works, so the release of that film would be the best possible time to release a game with the theme. Oh, and again, great music. The Chicken Run soundtrack is seriously underrated!
As for My Little Pony, while it will certainly never happen (And wouldn't sell well if it did) since almost no-one in the market watches the show, it would still make for a great pinball game. No, I'm not a brony, but the current show is on par with all the other great cartoons out there, and is just as enjoyable by both kids and adults as Spongebob or Gumball. Keep in mind, only the current show is. The older stuff is terrible. The current show also has multiple story arcs that would make for great rulesets (Collecting all of the elements of harmony or defeating all of the shows villains, for example), and as mentioned I'm sure some of the show's voice actors would be more than happy to do custom callouts for the game.