Something they could have, is a separate queue for teams, but not open all the time. Perhaps it's open in the weekend, and everyone can join it, from solo, duo, trio and up to a 5-man team. And you would still have the normal solo unranked queue.

Keeping it limited in time, would make everyone that wants to play it gather at those times, instead of spreading the players out across the entire week. This could keep the queues go faster and keep the mode active.