So I've been giving some thought to how to safely balance Demo's damage without over-nerfing it. Part of the problem is that Demo's damage in MoP is somewhat comparable to Destro's damage in Cata: there's a lot of little damage sources. A flat nerf across the board would likely leave Demo in a similar state to live Destro, where no one source of damage is very threatening. So, if possible I think it would be best to prune some of the less-significant or extraneous damage sources, and leave the good parts of Demo alone.
First, I think Meta's "melee" attack should be removed. While it started out as an interesting mechanic, it's currently just a vestigial ability from an old design that's eating into our damage budget. It's not a very interesting effect and, if given the choice between axing it or reducing the damage of another ability, I'd go for the axe. Alternatively, it could get a longer swing timer, have it's swing reset after a spell cast, and become unusable while casting, which would allow it to exist as flavor but preventing it from contributing any notable DPS (essentially becoming a built-in wand attack).
Second, there's Wild Imps. Now, I like the wimps as a mechanic, and I like them as a source for MC procs and fury generation, but the little guys make up a whopping 15% of our DPS. That is the single largest source of damage Demo has. While I kind of like that the little guys have some punch, I think it would be best if they took a damage hit. Additionally, it might be worthwhile to remove the cooldown reduction effect from Imp Swarm. While I'm fine with using Imp Swarm to game the system and stack 5 imps with a proc or cooldown, the cooldown reduction from haste makes it a definite DPS gain and a must-have glyph.
Third, if we're still over budget after removing "melee" and nerfing the wimps, I think that the majority of our remaining balance nerfs should go towards our DoT damage. I like the contrast provided by Demo's slow filler and fast nukes compared to Destro's fast filler and slow nukes, and if possible I think that should be maintained.
Balancing Demo's AoE is a bit tougher (as I haven't seen any AoE sims outside of the 'lock ones). Nerfed DoTs would certainly lower Demo's multi-DoTing, and there's the recent Void Ray nerf to consider, but if I had to guess then any future nerfs will likely happen to Immolation Aura.