A game with a truly completely fluid magic weaving system where you can casually levitate spoons around the corner and then liquify that spoon into a pool of metal and finally having a spoon-elemental emerge. Magicka comes really close, but even there you have pre-defined spells with specific effects in addition to the "3 stone 1 fire 1 arcane" stuff. I can't just magically slap on a conjured knife onto my fire elemental.
Bonus points if the magic system is gesture-based like in Arx Fatalis.