Journeyed there and back again
It's quite an interesting idea to use rap specifically. However, the relation between language and magic is a very old one. In fact, most real-world magic systems (Kaballa, alchemism, voodoo etc) rely on the user saying a spell / incantation just so, otherwise the spell is useless. In many magic-systems there's also the element of rhyming.
I guess I was aiming more for the idea of full on sentences for spells and a continuous use in battle. In Harry Potter for example it is just a 2 word spell or whatever and then bam. I like the idea of the pressure of having to make sure the lines rhyme and coming up with it on the spot during battle. They wouldn't be previous recorded spells with a specific standard for making them work. It would basically be a a freestyle the entire time.
Seems like it would take a lot of effort to make work and keep it from being broken though.