I don't expect Misty or Devack to ever happen personally. They're probably not going to do every single Dark Signer, and they're the two easiest to cut. (Also, they'd have probably saved some of Misty's Reptiliannes if they planned to release her, she's basically only got Dragon Queen of Tragic Endings and Ccarayhua at this point.) I'd expect Greiger next if he's happening at all, otherwise Roman.
To me, it does not matter. The question is what will the next unlock event be like? A Raid Duel like Kalin's and Rex's or just a regular unlock event to collect Dark Signer Vestiges like Carly's?
Every time random people say: "we won't get the other characters" I laugh, because that's a huge lie.
They can add whatever they want. "We have all their cards" is another invalid argument. They added BLAIR, SCUD, LUMI & UMBRA, even Mokuba dsod and the guy NEVER dueled.
Can you please explain the reasoning for Scud, Mokuba DSOD and Joey DSOD (who did not use cards during the movie), or Tristan who literaly played one-third of a duel in the whole series? I believe we can potentialy get all the characters. And only Konami could give us a definite answer.