The Starcraft 2 ending didn’t leave any room for a Starcraft 3 with Starcraft 2’s cast of characters. Nothing like Amon will ever threaten the Koprulu Sector ever again, so Ascended Kerrigan will have no reason to use her Xel’Naga abilities beyond disguise. I could just imagine it. A young woman appears and helps out in some minor way then disappears. You suspect who it could have been, however you don’t know for sure; and Blizzard will never say who it was because it’s better to leave some things mysterious.