How about this: a temporary buff, like Spinning Heel, that summons the wyvern for 20 seconds, and a similar cooldown (2 mins). While out, it will auto attack your target.

As for special attacks, it can do breath attacks upon combo completion, or you could have its breath WS as a final hit in a combo (instead of Chaos, RoT, etc).

Later on, have an ability or perk that lets you take it's TP for yourself (or when it unsummons).

Idk, just brainstorming. Save the tomatoes.