Well, regarding extra content and DLC, I think the big problem is what's available from the original Super Mario Bros. They gotta find four equivalents really, whether it be enemies or locations. They already made a couple sets for the early games where there were none, I know they made some new sprites and things like Bowser Jr. in SMB and SMB3 style, but I think someone like him is an exception. I don't think they'll feel like making new assets for any random thing.
I do love to see more content added though.