I'd love for characters to return, I miss a lot of them. I'd love for Dawn to return most of all as she was my favorite heroine in this series, I don't really like Misty or Iris so I could live without cameos of them. I admit I do miss Brock finally, so many years I wanted him gone.
If Dawn has to return, they definitely should make sure she does so only in a region that has Contests, whether Anime-exclusive or otherwise appearing in the games. Her BW cameo was just stupid and pointless.
As far as Misty and Iris, I won't comment on Iris, largely because she at least IS pursuing her goal, but I do think Misty should return, if for no other reason than to actually have her pursue her goal yet again, and NOT be forced into a Gym again. If she must leave, it really shouldn't be to run a gym again. She wanted to become a Water Pokémon Master, something akin to the Elite 4 or even the regional champion, in other words. A Gym Leader doesn't suit her goals. Did Ash become a Gym Leader to become Pokémon Master? Heck no! You may not personally like Misty, but at least understand that she really needs to be able to actually pursue her goal rather than be forced into a gym and be a punching bag. It would be like if at the end of AG, May, just as she was about to set out for Johto, gets an emergency call from her dad telling her that he's either become extremely injured and/or falling under a grave illness and has no other option but to assign her to be the gym leader of Petalburg City Gym, meaning she's forced to put her goals on the side burner to take care of it, and she cannot escape from the Gym afterward, to give a good example. I always hated what they did to Misty at the end of Johto, especially after AG basically made most of the Gym Leaders out to be wimps. I don't want Misty to end up being treated like Jessie James and Meowth were in Episode 3 of the entire series where they lost to a Caterpie despite having the clear advantage.
As far as Brock, any coolness Brock may have had was killed in Kanto, with Johto onward, they just turned him into Glenn Quagmire from Family Guy. If they are to bring him back, they need to do at least one of two things: 1. Have his goal, whether it be Pokemon Doctor or Pokemon Breeder, actually be FOCUSED on, and 2., and probably most importantly, they should either cut out his womanizing gag, or at the very least tone it down significantly, as his gag is completely inappropriate for the target age.