Worldfire is legal in Commander again, so I thought I'd throw together some bad ideas to see if the community can make anything of them.
Feel free to post your own!
Combo #1: The GG EZ
Worldfire + Chandra, Awakened Inferno: pretty self-explanatory and simple to splash into a Big Red deck. Chandra herself is uncounterable, and her +2 gives each opponent a damaging emblem that will not be removed by Worldfire or other methods.
Combo #2: O-Ring Strats
Worldfire + Oblivion Ring/Aligned Hedron Network + Goblin Chainwhirler/Dagger Caster/Lavaborn Muse/Warchief Giant: bit weirder. Use O-Ring or Aligned Hedron Network (if mono-red) to remove any of the above. When Worldfire's effect resolves, the O-Ring effect triggers, returning Chainwhirler/Caster/Muse for lethal/returning Warchief Giant, who has Haste and Myriad so he can attack all players at once that turn.
Combo #3: WotC's Folly
Worldfire + Spellweaver Helix + (Dragon's Approach x 2): A combo that folds like paper but is INCREDIBLE if it goes off. Spellweaver Helix, which normally would not do much of anything in EDH, now has a grim purpose alongside a seemingly harmless little common. First, get Worldfire and 1 (or more) Dragon's Approach in the graveyard. Cast Helix, exiling both Worldfire and Dragon's Approach. Cast Dragon's Approach. Helix puts a Worldfire copy on the stack. Worldfire copy resolves, reducing everyone's life totals to 1. Dragon's Approach on the stack resolves, dealing all opponents lethal.
Combo #4: Graveyard Grenade
Worldfire + Dragon's Approach/Fiery Confluence + Izzet Chemister/Ignite the Future/Mizzix's Mastery: By exiling Worldfire and an instant or sorcery that deals damage to each opponent, you can use Chemister, Ignite the Future or Mizzix's Mastery to put both on the stack, ordering them so Worldfire resolves first, knocking everyone to 1, and then the damage spell resolves afterward, dealing each opponent lethal.