There's a bunch of stuff that I wish was better in my submission, but most of it I wouldn't say is something I regret per se, just stuff that had to be compromised for time reasons.
I do regret making 50-50 grinds worth points though. In hindsight I think it would have made more sense to make them something that will keep a combo alive but not grant points, like I did with manuals. (especially since manuals are actually harder to pull off)