Syndra is one that I've had trouble working around as a support. I recognize all the potential there, but for some reason I either go 4/0/0 in lane (stealing all the kills) or I go 0/5/1 in lane. I just need to practice more with her I suppose.
Cassiopeia has been one of my secret picks in the past, pre-rework. I don't know how she would fare now since her base dmg has been reduced.
Xerath is a really, really good harassing support who can secure kills behind the enemy turret if they escape the ADC's range without risking turret dmg. He lacks utility outside of a slow and a single target stun.
Tanky support picks that go outside the usual suspects and that I like are Nautlius (on whom I highly recommend MS quints instead of gp10s) and Sejuani. Nautilus has peel, engage, initiation, and a lot of beef with that shield. Sejuani is a higher risk pick obviously since she has no long range engage outside her dash, but her early AP dmg is unexpected and she has a strong, huge AOE slow that serves for lockdown pretty well.