Fiery Confluence seems sweet, given how flexible it is as a red burn spell.
Orim's Thunder Is similar to Fiery Confluence, but lets you hit enchantments. It's not red though, so you wouldn't gain any life.
Aftershock just turns into 4 mana destroy target creature, artifact, or land.
Flame Rift, Sizzle, Price of Progress, etc just do a ton of damage and gain you a lot of life.
These are just some ideas off the top of my head, but they seem good to me.
Edit: Righteous Confluence is also sweet.