Extended Call Lightning? 3d10 lightning damage each round, for 20 minutes (200 rounds). Kind of blows away Witch Bolt, but at a lvl 3 slot and 1 sorcery point. Empower it if you want.
But so does Extended Conjure Animals. With Raven Swarms you're potentially looking at 8x 2d6, +3 to-hit attacks, for two whole hours (1200 rounds). More with AoO's. Level 3 slot and 1 sorcery point. More Raven Swarms for higher slots. Pretty damn efficient in theory, if not really in gameplay.
Their answers do also lend weight to the Hex/Eldritch Blast (or Scorching Ray) controversy, on the side that these are single target if used against one target, therefore the +d6 necrotic damage is given against the hexed target, even if you twin them.
There is also nothing stopping you twinning Hex on the same target by these rules, targeting two creatures at first, then bouncing them both onto a single creature once at least one of the the original targets are dead. But can you give disadvantage on two different attribute checks with this? And do you get +2d6 of necrotic for any of your attacks, or just +d6? Do they stack?
A can of worms indeed, this whole single target debate.