I feel that if the region is connected to Kalos, Sycamore may act as professor in it, too. Or if we get a new professor, Sycamore could still have a larger role, similar to how Oak played a small part in the story of gen 2 (and introduced us to it, leaving Elm as our only prof. to not welcome us into the world of Pokémon).
Then again, if the region is connected to Kalos, we'll probably see a lot of Kalosian NPCs, as I feel Game Freak would probably not make, for example, a completely new set of 8 Gym Leaders when they could easily reuse most of the ones they already have.
On another note, this is something I brought up in some other thread but I guess the same idea applies here, too. Some characters work really well as "cameo fodder" because they aren't really very in-depth, so just appearing as a cameo doesn't really "ruin" their character. But then there are some characters who are very in-depth, and just appearing as a cameo could leave a bad taste in your mouth that they weren't utilized to their full potential.
For example: Cynthia. Awesome, but not very deep character-wise. So her appearing as a cameo character in BW feels appropriate because she can battle you with her powerful team and have this sort of "aura" of just how cool she is. Nothing about her character is "lost" and she remains "herself."
On the flipside, Looker. A character who, to a good number of fans, is known for helping add depth to the narrative he joins in. So him appearing in ORAS just to give you the Audinite, with amnesia for absolutely no reason whatsoever, feels almost insulting to his character. Why is he there? Why does he have amnesia? Why does he have the Audinite? Fans ask these questions because they've come to associate narrative depth with Looker. But in this case he was demoted to "standard NPC:" any nameless NPC could have given you the Audinite and nothing would have changed about the scenario. So unless Sun & Moon play off the plot point of Looker having amnesia, the whole scenario was pointless and Looker "demoted."
So when it comes to certain characters, especially those associated with a lot of narrative depth, they can't really be "cameos" without degrading them in some way--and personally, I'd rather those characters not appear at all than appear in a way that is completely disrespectful to their character. I know that Colress was used as an example, but playing off of that, he'd have to be a pretty major character to justify him being in Sun & Moon, otherwise it's not really gonna feel like Colress--rather, he'd have to have an appropriate reason for being there. This applies to other major, in-depth characters, too.
There are definitely appropriate ways to incorporate such characters as mere cameos, but the key thing is "reason." Nobody wants to see their favorite character show up only to be upset by how they were handled.
Like, I would love it if, for instance, this was the region where N and the BW protagonist finally met up with each other (bonus points if there's some sort of data retrieval function like in B2W2 where you can have the protag be the same as yours in BW). But that kind of thing would require at the very least a heartfelt scene to play out, not just have N stand there doing nothing.
