You mean, combined spells, such as spectral blast and hydra? Balance already has it.... but I see where your going with this, I think it would be cool to like mix school spells together in some way. Sort of like hatching, but with spells
There is a school that is all schools combined. It's called Balance. It has shields, blades and traps that will effect multiple schools and a couple of blades and traps that will effect any school. They also have three spells that will use one of or all three of the elemental schools. It would be nice to see a spell that would use the spirit schools but I guess we can't have it all.