I wanted to make a character like that as well, but then I fell in love with my sorcerer. I was going for the support line in DK as well.

Apart from the DK, you also have :

The werewolf line. It's broken right now... but when/if it's repaired, you have :
- pack leader (allies gain ultimate faster)
- rousing roar (increase weapon damage of nearby allies)
- call of the pack (allies remain in ww form 3% longer)

There are some buffs in the resto staff :
- combat prayer (armor, spell resist, damage increase)
- ward ally (a shield)
- siphon spirit (allies gain magicka and health when hitting target)

Undaunted guild. Almost all spells have a synergy.
- necrotic orb (allies restore magicka)
- bone shield (a shield)

I haven't seen much outright buffs aside from the DK, but the siphoning line in NB is healing from your dps so it might fit as well.