I wouldn't mind seeing Catalysts not being usable on Very Rare enhancements (attuning doesn't do anything for them)
Now, for a change that would require more work. If a Catalyst is used on a PvP, it doesn't attune, but gets changed to your level. (1-50)
For other stuff, a De-Attuning method would be nice. Heck, you could just make it a second application of a Catalyst would make it your level. So, I'd see someone using a catalyst on a low level until they hit 50, then de-attuning it. Maybe only make this option able to be done at 50?