This whole thing's got me excited. It's an update I didn't even know I wanted for a collection I already thought was notable for having the most games in any official Sega Genesis collection yet. Apparently, some people said the Sega Genesis Collection's launcher doesn't work well in Steam's Big Picture mode, which I never used, so I'm sure this probably fixes that and then some.
The Steam Workshop support for romhacks is especially interesting to me, as this makes Sega the first game company to embrace fanmade romhacks in an official collection of their classic games. That should make it a lot easier for people to get into them, at least for a few kinds of games. Now Sega just needs to add a couple more games to the collection. I'd suggest Monster World IV for one, since a translated version already made it to consoles, and if possible Castle of Illusion since the Genesis game was brought to PS3 alongside the remake anyway.