The update incorporates 192 levels, difficulty-based map selection, and 10 central characters from classic series like Mega Man and Ninja Gaiden. Each character can be re-skinned with a library of sprites from their past along with a few surprise inclusions. You can pummel goombas to death with the ice rod as the old man from Zelda 1, for example.
The conclusion of Version 3.0 allows Exploding Rabbit to pursue development of Super Retro Squad, what they describe as "a combination between Super Mario World, Mega Man X, and Super Mario Bros. Crossover."
Their previous Kickstarter campaign only drew in enough funding for Director Jay Pavlina to work full-time on the game, however, so they've launched a new fundraising effort. Considering they've released three versions of Super Mario Bros. Crossover for free, it'd be awful reasonable for fans of that project to help them out.