The following comes from a Reddit AMA with Daniel Smith of Gray Fin Studios...
Online multiplayer was dropped quite a while back - because it was such a headache in terms of creating new bugs/issues. Online leaderboards for Arcade mode were another thing - and were dropped pretty late - i was going through a period of feeling like the game would never be finished, and it just seemed to be adding complexity, so i had to be ruthless (it did make the Steam version however, where it was simpler to implement). Hopefully i can add these back in an update.
I plan to do an update right away to address a few bugs, and balance some difficulty issues. For new features, it really depends on how well the game does, especially for bigger stuff. I have plans for new game modes/levels, but we'll see!
Stuff like off-screen play, using alternative controllers for player 1, and the aforementioned online leaderboards will likely be added though.