For a Ganondorf Echo I nominate Urbosa.
She's the most relevant 3D Zelda side character choice of the Gerudos and Urbosa has the height, sword, and magic that would line with Ganondorf pretty well. Just change Ganon's magic effects to lightning, as well as borrowing from Pikachu so she can have some attacks with electric properties. Urbosa could be a Chrom like Echo and have Ganon's side b replaced with a projectile, Warlock Punch changed to either a shield bash or a quick electric punch that stuns (basically Ryu's down b with electric effect) and her down b is Pikachu's.
I would say the two things that are against her chances for Ultimate would be if Sakurai wants to add more BotW content than what he's shown so far (assuming she's not already playable), and if Sakurai would add her to the roster without the rest of the Champions as newcomers.