Most of my ideas are more along the lines of 'what if' scenarios, and most of those are in season 2. season 1 is pretty legit, except for killing Presea, but if they could have gotten away with it if they'd just handled season two better.
Flash backs to a few scenes between the two seasons would have been awesome, I also liked the small flash backs we get to right before season one, more of those would have been cool.
I've argued this before, but I really never liked the idea of Debonair. Either they should have followed the manga closer and not had a real 'big bad' or all three of the girls should have created a being similar to Nova and they were the main problem in season 2. I guess they figured they couldn't fit that much character development and drama in one season. And they were too busy beating Hikaru with an angsty sick.
Really I just hate Debonair, she's the exact trope they were deconstructing in season one with Emeraude and Zagato, why fall back onto it?