Sounds like they've run the gamut as far as the BC tricks go that they currently have available. They could always find a new way to make this work on titles that didn't work before, or they could develop new techniques that could apply to a different set of games. Just allowing games from 4 generations of consoles to be played on one box is huge in and of itself. The bonus upgrades are just icing on the cake.
If I was going to lament any particular part of BC on Xbox it would just be that it is too bad that more of the OG Xbox games didn't get preserved. But, I understand that the licensing and all that was probably a nightmare on those.