Fairy Tail Dragon Force

I watched episode 67 (i think) of fairy tail recently and I thought about making a dragonslayer build in GURPS. I first thought that a dragonslayer, when they take fire (or whatever their element is) damage, instead regains that damage as FP and HP. My idea of Dragon Force (where the caster takes on the physical properties of a dragon) was that they could only cast in when the GM considers it appropriate, i.e when you're losing against a boss. FP costs double, all spells count 1 round of charge as the maximum bonus from taking rounds to charge up, damage is tripled, and you dont expend FP during the fight, but instead lose it all in one big chunk afterwards. Anyone who's watched the show got any suggestions/improvements?

Fairy Tail Dragon Force