Someone on production said, after the Ro episode dropped, that someone "even bigger" is coming, and he can't believe it hasn't been leaked.
In terms of someone who would fit the show, another cloned Weyoun showing up would make logical sense. Certainly Jeffery Combs is always up for a role in Trek, and he would be a logical person to introduce if you wanted the Dominion to be back. I don't think Combs returning for another show would be a big shock, any more than seeing Martok though - they have close connections to fandom after all.
Lots of DS9 characters would make sense in a Dominion-focused episode, but we all know who the giant shock would be - the man who walked away from DS9 and had virtually nothing to do with the franchise again. Who had unresolved bad blood with Picard. Who might make sense if (as has been hinted) there's some Pah-Wraith stuff going on with Jack as well.