Same as the others who replied so far, and with the acknowledgement that additional leaders are probably ridiculously unlikely at this stage of the game.
But, for the sake of the topic (and HW3 in another 3-5 years maybe, 343i? come dig this out of the archive later):
Tem'Bhetek, The Prelate - basically the Spartan equivalent of a San'Shyuum (the prophets). Super fast & strong melee type leader, detailed in the Shadow of Intent. I advocated for this character as a leader in a much older topic like this and still think he'd be fantastic.
A Yanme-e (drone) - I think this would work similarly to Yap, but via the air.
Spartan Palmer or Spartan Locke, with a Spartan IV hero/drop similar to Omega team in functionality (Alpha 9 if the latter, maybe as ODSTs before a pricey Spartan upgrade?)