I wouldn't really consider Burger Knight as a wishlist character, he is in a bigger league than say King Hippo or Waluigi.
IMO his chances are somewhat good since he is after all the main antagonist from Ike's game, plus Ike will probably return so I could probably see him as the 3rd FE rep alongside Ike and Marth.
His chances of course would considerably drop if we indeed to vet a new FE lord announced and represented such the way Roy did in Melee, but other than that I think it's safe to say that he will at least be considered and isn't necessarly out of the picture just yet.
Tbh Masked man really doesn't stand a chance compared to The Black Knight, The Zelda series already has sufficient reps and with Gharaim coming around in the new game; he is the most probably
if we indeed where to get a 5th Zelda rep.
As for Medusa, shame on you!!
She definetly has realistic chances considering she probably going to be the main antagonist villain from Kid Icarus Uprising, though if she doesn't I could easily see Palutena in as well, heck Magnus would be awesome.