In terms of Magic spells, I hope you can mix already mixed spells. For example mixing the Fire+heal Spell shown in the presentation with like a lightning or a blizzard, or maybe even another mixed spell which creates an even ridiculous spell. That might be a bit much to ask for but who knows? Imagine Mixing a Firaga with a Thundaga and getting like a chain firebolt or a thunder which causes a long lasting paralysis or something.
Oh, shit, while writing that I just realized what system they should use...the Magika/ Magika 2 system where you select different elemental and base modifiers which allow for interesting spells.