I honestly can't see an indie becoming a playable character. In all honesty Super Smash Bros. is kind of seen as the ultimate video game crossover by the general audience, so a character getting in would feel like an honor for the company who owns it. No surprise why many third-party companies would like to see their mascot in Smash Bros. Obviously getting a playable spot in Smash Bros. is never an easy thing for any third-paty character, as it's supposed to be a giant Nintendo crossover in the first place after all. Due to that third-party additions are always going to feel like something special, and I don't think any indie, regardless of Nintendo support, has enough star power to feel special like that alongside Mega Man, Sonic and Pac-Man. Realistically the only way I'm seeing an indie become playable in Smash is if there are plans from Nintendo to buy the company who owns them (like Monolith Soft) or to co-own the IP with them (like Game Freak). In other words, they would no longer be "indie". Plus we already have the CommanderVideo trophy cameo as an indie rep anyway.
I think the top contenders for a spot as Smash's first third-party DLC character ever are Banjo-Kazooie (now that they're within the realm of possibility), Rayman and Snake, with anyone else being highly unlikely, especially indies. And even then I don't think any of those three are likely to be honest. I'd LOVE to see Rayman or Banjo-Kazooie in Smash, but I think I'm letting myself to be disappointed if I think they're likely.