Any character who was in AoC but not BotW (Sooga, Terrako, Astor and the new version of Ganon come to mind) have to have a reason for not being in BotW.
So Linkle would be a bit awkward, but it would have been awesome.
A level called Versus Calamity Ganon popped up so I'm going to attempt that later today!
I figured out a "cheat" for it, if you have the DLC: Use the Monk. He has a DLC move where if you have all the symbols (and there's a move that will instantly fill all your symbols, 5-6 Y -> 3 X) and you...
So I've completed ALL available challenges that aren't DLC (including Blight Ganons Resurgent, Mysterious Arrivals, and Versus Astor) and I STILL haven't seen the final character. I'm completing the Divine Beast challenges just in case before moving on to the DLC challenges for more materials...