I'm pretty torn on gen 5 characters getting cameos. Like, they're my favorites by far, but I wouldn't want them to feel... ruined, I guess?
I mean, since they are the most developed and story-driven of the NPCs, you can't really just throw them anywhere without giving good reason. Why would Cheren be in [new region] when he's supposed to be working in his Gym? What kind of assignment did Juniper give to Bianca to make her leave Unova and go all the way out to [new region]? Maybe N could be justified by having it be just a stop in his journey of self-discovery and looking for the BW protagonist...
But it still needs to be a meaningful cameo. Like, Looker in X & Y was great because the Looker Episodes were the most developed storytelling in the entirety of gen 6. But the way they just threw him into ORAS was disgraceful, in all honesty. He's really the only non-gen 5 NPC who is particularly impactful narrative-wise, and they demoted him into a role that any nameless, generic NPC could have fulfilled. I was honestly very upset to see Looker portrayed in that fashion.
I would not want that happening to any of the gen 5 NPCs. Sure, I would love to see them again, as they are my favorites of the entire series, but I wouldn't want said cameo to reduce the character into a useless NPC. Part of the reason why gen 5 was so great was because they weren't generic, useless NPC's! They would need to be fairly meaningful, substantial cameos in order to prevent that.
Characters like Cynthia... while Cynthia is most certainly awesome, she didn't have a whole lot of growth/development as a character, nor a particularly complex backstory. She's just super cool. Which actually makes her a good candidate for a cameo because as long as she is a challenging opponent and is placed in a setting to enhance her demeanor, she'll be portrayed appropriately.
That being said, characters who I know I wouldn't mind cameos of in that regard would be Cynthia, Sycamore, Steven, Wally, Lance, Lorelei, even Blue, among others, in no particular order... and maybe even Diantha, to be honest. Assuming we don't get to visit Kalos in Sun & Moon's post-game, having Diantha make a cameo appearance could actually be beneficial to her character.