Fire bomb | Fire MortarWow, I'm surprised this has the most votes.This would make be the sixth spell with two of them, Rain of Fire and Fire Bolt, already being extremely similar cards. I like the spell but find it considerably less useful than the other ideas due to Fire already having a few spells.
Spontaneous Combustion | Spontaneous CombustionI like how it uses pillar amounts to become a spell with a really nasty damage range that can take care of large creatures like Armagio instead of just being a bolt clone. Sort of reminds me of a last option high damaging self destruct except leaving you without quanta instead of causing damage to you.
Charcoal Stallion | Fire StallionHaving the nice high damage/low health theme you see in Fire while having a nifty ability that can destroy permanents. If I had another vote, this would be the card I'd choose.
Lava | LavaI'd love to see more cards that affected the field directly like Flooding and Lava. Flooding never saw much use since very few decks use the bottom and top row and playing multiple ones only increased the upkeep, but Lava seems to lack those problems by going side ways and having great synergy between itself and its upgraded form. Did I mention it would be the first permanent in this element outside of weapon/shield slots?