We should not discount Jean-Luc Picard yet. He is human, and humans have a way of showing up when you least expect them.
An in-joke, of sorts, about Denise Crosby, whose character Lt. Tasha Yar was killed in Skin of Evil (01x23)
, but managed to show up again in Yesterday's Enterprise (03x15)
, and now has shown up a second time playing Commander Sela .