You stole this idea from a good friend of mine.
Whiplash posts a ton of ideas he's had for HOTS, including this entire Molten Core map idea. You even stole his mechanics.
http://us.battle.net/heroes/en/forum/topic/12675389453
Exhibit A, he called this arena "The Incinerator", with a description on page 2:
http://imgur.com/uqBngA7 Objectives:
Every few minutes, lava will rise in set locations and most importantly the charred circular area in the top center of the map, and Ragnaros will spawn. Defeating Ragnaros will send him into a frenzy and he will damage ALL buildings with a rain of fire dealing damage over time. Outer ring bases will take 50% health damage total, inner ring bases and nexus' will take 25% damage.
After Ragnaros has been defeated he will submerge, and the lava around the map will diminish for a few minutes making the trapped SCV's accessible.
SCV's who were trapped by the lava will be able to be rescued using the mercenary beacon mechanic, this beacon appears once your team has killed the small fire elemental that was left over in smoldering lava recesses. Once rescued, they will head to your team's base and begin to mine minerals and gas up to a certain amount. SCV's are invunerable and cannot be killed.
Once an SCV has mined 50 minerals they will speed off to repair the most damaged tower/fort/keep/nexus in their base up to a % that is always less than the total damage Ragnaros fires have caused. Once the repair is finished the SCV will despawn much like the M.U.L.E talent.
TLDR: Every few minutes lava rises and Ragnaros spawns, when Ragnaros is defeated or submerges he will shower the map in flame, rescuing trapped SCV's to repair your base is the key to victory and outlasting the flames.
ANYWAYS...just sad to see this idea stolen. He has a ton of great other ideas too, if anyone in the community is interested!