One of the other players is making a Polearm Master Fighter, so I'm thinking of building a Repelling Blast-spamming Sorcerer/Warlock to help the fighter to block off choke points. Polearm Master allows you to make an OA whenever someone enters your reach, and together with Sentinel that drops their speed to 0. So the idea is to push enemies into awkward positions where they have no choice but to run back into the Fighter's reach. It's basically a light 2-person version of 4e's awesome Polearm Momentum build. This trick doesn't help with ranged enemies, but for them I'll instead try to push them into the Fighter's reach (or maybe get Grasp of Hadar to pull them?).
Of course, I'll be using the usual Quickened Spell trick to fire off a bunch of Eldritch blasts.
I figure it would also be nice to take advantage of the Sorcerer's Twinned Spell and Haste to turn the Fighter (and whoever else has at least 2 attacks) into a killing machine.
Since I'll just be using Twinned Haste and spamming my Quickened Eldritch Blast, the rest of my build choices are pretty open-ended. I figure that the Hasted Fighter will probably be doing the bulk of the party's damage, so I'm thinking of filling a supporting role to some extent.
What other support-themed choices would be good fits for this build? It seems like the usual wisdom with an Eldritch Blast build is Warlock 2/Sorcerer X. Should I stick with this or would it be worth taking more Warlock levels (or levels in a different class entirely)?
All official sources and Unearthed Arcana are allowed (with DM permission).