Apologies if others have seen this, but if you go to this video here, around the 3 hour, 5 minute mark, it shows someone playing Firefight Terminus as the Arbiter.
At around the 3 hour, 8 minute mark, you can see some of the Arbiter’s leader powers, including:
I see Stasis Mine, Plasma Bolt, Conduit of Rage and others.This includes an unlock for a buildable phantom from your base.