Vel'koz is not a support the same way Annie isn't a support. Even by that comparison, Annie's a better support than he is. Vel'koz is a spammy, long ranged immobile mage with some CC who's main asset is damage. No, i'd like the Void to have an actual support with a kit designed around helping others out, Void-style.
Something... thematic like 'take a bite out of an ally champion, dealing X damage to them but making them enter a frenzy, increasing their damage by X and attack speed by X.'.
Ultimate would involve some good old hard CC, Q a skillshot slow like all 12 last released champions. Passive could easily involve dead minions serving as food (extra health recovery).