As much as I love the character select menu (Only saying this because this may cause it to change if implemented) as it reminds me of Marvel: Rise of the Imperfects (somehow),
I think character unlocks would be sweet. For example (if there isn't a bf assist) Boyfriend could possibly be unlocked by completing a run with Girlfriend, perhaps beating the BF bot with her special?(Final hit with the beatbox, to add a bit of challenge)
And that is just one example. There are a couple of characters that people would definitely love of you were to add, like Tankman, PBot(though there is probably already an assist trophy) Maybe Alien Hominid, and the prisoners from Battleblock Theater(I mean those little guys can fight).
But each ot those unlocks would be accessed by completing an action that somewhat related to the IP they come from, even if just vaguely.
(Also, Portal Panic and Newgrounds Rumble are some pretty good Newgrounds allstar games it would be cool to see modes based around those or even remakes if possible even though I don't think you had anything to do with those original games.)
Honestly ignore that last part.