There's no reason not to have online multiplayer at this point. Personally I'd love to see them add to it as well, like maybe, in addition to co-op, there's a race mode to see which player can reach the end first.
Best 2D platformers I've played but...I think the 2D platformer genre is dying. NSMB keeps trying to evolve, but there's not much room left for it. I would love to play 2 player where you don't have to share the same screen though.