BLM doesn't need an AoE buff. Speaking as a BLM main since 2.0, we are in a great spot. Seeing my parses in A2S is enough to know our AoE is fine. SMN has their AoE niche and their mobility niche. BLM ST numbers are not even correct half the time. I hit a 16.5k Fire IV Tuesday during raid. 16.5k. That's not even right. A BLM who knows what they are doing will do just fine with mobility as well. It is a little bit harder to maintain proper uptime with our long casts and restrictive timers but I seem to manage just fine. The tools are there for us to use, you just have to use them.
As for the OP's original idea, I actually like that. I say that as being a person who cringes every time someone says our AoE needs to be buffed. The idea sounds pretty decent to me. My only thing is I absolutely think we cannot be comparable to SMN in the AoE. I think our numbers are perfect.