I would associate a spell with an amount of Mana and a difficulty and so the harder it would be to perform, the more mana and score on the dice you would need.
But yes you can come up with spells on the spot and if they are reasonable I will allow them. 
But the more powerful a spell is the worst it will backfire if failed (say if you got a one on the dice)

And if you want to balance melee with magic that would be fine, I find it best to balance most skills anyway.