You know what, I forgot to add a final important criteria to the delta (which I'll edit now): A Delta no longer retains the derivative name, logo/emblem, or costume as their Alpha. Hence Power Girl and Huntress would be Deltas, but Supergirl and Batgirl remain Betas for as long as they carry on the name/emblems of their respective Alphas.
Steve Trevor is never going get his own ongoing series to develop his own independent supporting cast and his own independent personal rogues gallery, with a new origin independent of Wonder Woman. He will never be an Alpha, or even a Delta. He'll stay a Beta and that's OK. His star goes as Wonder Woman's star goes.