Places where the standard prism are of little to no use: Damage-over time spells
If a teammate uses your prism If a teammate breaks it with earthquake A few cheaty bosses who can remove stuff from themselves
This is my idea: If you have cast a blade type prism, then cast the spell that would use it, your spell will instantly transform into the same exact spell, but the opposite school (any after-effects would also be changed , too). Of course there would not be blade type prisms for balance or any astral school.
Disclaimer: W101 would have to create hundreds of new animations, so I don't think this will happen. But I can wish, right?