At 36th level, Destro locks get Havoc.
That causes their next three spells (or next Chaos Bolt) to deal equal damage to the Havoc target.

If you had four locks, each lands a Havoc on one target.
Then you attempt to Chaos Bolt that target.
That could be the damage of eight stronger spells, all at once; and since Chaos Bolt is always a critical strike, that could be pretty nice damage.



I also like Demo, because of the pets...
I like Wild Imps and Felguards.
You might not need Felhunters, if you have a Druid with you for heals.
Still, you could go with several pets, and hotkey abilities you like.

Metamorphosis with the Apothesis glyph could be nice.
Basically a tanking form, when you need one.
But you don't need to be in Demon Form the whole time.

500% AoE size (normal + 400% extra) could be a rather nice 90th talent...
Seed of Corruption, against stacked groups.
Immolation Aura and Hellfire and have the Felguards do their AoE axe thing, if you're swarmed...