Firemage currently is one of the worst AoE Classes against anything beyond Cleave. And for Cleave you need big rampup and its only mediocre at best.
Frost is good for AoE because of its mechanics, while Arcane is good simply because it scales with its mastery. Why cant Fire AoE Scale with Mastery too?
Take away Arcane Explosion from Firemages and give them a new Version of Blastwave!
It shouldnt be a 1:1 change that would lead to spamming Blastwave every GCD instead of Arcane Explosion. Make it work similar to the D3 Explosive Blast with Chainreaction Glyph.
Blastwave - Instant - No CD
2 Seconds after Activation you cause a fiery Explosion that deals x Fire Damage every 0,5 Seconds for 4 Seconds to all Enemies within 15 yards. Deals increased Damage to targets afflicted by Flame Strike.
Of course it can't stack so you want to activate it 2sec after it starts dealing DMG for max Damage. And you want to put down a flamestrike before. (activate Blastwave -> cast Flamestrike -> do something else for 1-2 GCDs -> activate Blastwave).
It currently just sucks to try to do any AoE as a Firemage because 95% of the other classes will own you in Recount as soon as there are small windows of AoE DMG.
Now that Combustion is nerfed to the ground just let make it spreadable to all targets again... even if you get a good 100k DMG Combustion it wouldnt be that overpowered because Demo Warlocks and Arcane Mages do the same numbers for AoE without preparation.