[X] Heart of Fire. You're sure that Altria will be so jealous like it.
The other aspect to this is that if we can naturally generate fire/are fire aligned, we may well be able to pirate Fire Bending from Zuko. Having a significantly boosted ki based set of offensive techniques available would be very useful. Ki use appears to be much lower profile than magic use, which is very important on the Hellmouth.
Even if this is the case, it shouldn't give us a whole new skill set to train, as it should build on top of one of our existing Prowesses, hopefully Spiritual Manipulation, which we have to raise anyway. We can simply steal the techniques from Zuko with Dicination next tournament.
Choosing this option does not necessarily make us fragile, either. A Fire Shield type spell would effectively make us a much harder target, as it would deter many physical attacks and destroy/damage many projectiles. Significant Fire Resistance/Immunity would massively protect us from a whole class of attacks, which, combined with using magic to shape our environment, would make us much harder to kill, as only similarly fire resistant enemies could approach us.
The killer application of this is, of course, vampires, who this would render a much lesser threat.
I wonder if a ritual consumption under tailored conditions supported by the right reagents would allow us to optimise what we received.