If we are speaking on mvc2 specifically, the points Rainfall made I think is basically everything people could want. Maybe an updated training mode would be nice with recording slots, character specific switches, etc, since mvc2 training mode specifically has a really trash training mode lol
I look at how GGAC+R on steam was updated over time and see all the praise it receives for it's online and offline features and can only hope that those developing new ports would follow in that path. It would be nice if there was a tutorial or something to get people up to date quickly in-game but that will never happen.