That's your problem, you're relying too much on the G.E.M. series as your only evidence that Misty is coming back to the anime when really they're only being made to celebrate the 20th anniversary of Pokémon thus lots of new merchandise of elements from the first generation and nothing more. Mallow will definitely be our new female companion (and possibly the only captain on Ash's group) based on the leaked concept art, and I'm going to enjoy every bit of her character (unless she's another Iris).
If the G.E.M series was only being made to celebrate the 20th Anniversary, why release it now? The 20th Anniversary of the Anime is next year, NOT this year, that's for the games, and the G.E.M. Series was definitely geared more towards the Anime front. Had I been the guys doing the G.E.M. Series and we were gearing those to market the 20th Anniversary, I'd make sure to release them on May 2017, NOT May 2016, since that's for the games' 20th Anniversary, not the Anime's 20th Anniversary since it's not even the anime's time yet, and logically, if you're going to release anime-related material for the 20th anniversary, you release it of the 20th anniversary of the product said merchandise is actually geared towards (in other words, releasing it on 2017). Now, if it were the Bandai Gym Leader set that included Misty, you'd have a point since I know that wouldn't be enough to indicate a reappearance, but not with this. Besides, Misty didn't even GET any new products even during the 10th Anniversary of the Anime (which was during DP), so it's still an unusual move even taking that into account. They still focused on the current cast, not the Kanto cast (in fact, the closest she's gotten to a true reappearance during that time or any focus at all for that matter was in the Japanese dub for the Mastermind of Mirage Pokémon).
And it's not just the G.E.M. series, either. They also made the Alola region, which was modeled after Hawaii, an archipelago. There are literally plenty of locales they could have chosen from, whether it be any districts of Japan they haven't already used, or otherwise some American locale or European country they have yet to use, or even a third world country they could do like China, Vietnam, or even South Korea. Yet they specifically chose a region that was more water than land? Sounds fishy, to pardon the pun, especially given the timing of its expected release being around the 20th anniversary of the Pokémon Anime (the only Core-series region we've gotten that was more water than land before now was Hoenn. Most other regions, even if they took place on an island, had more land than water for the overall map). And it's not like the games didn't get influenced by the anime or influence the anime, after all. Don't forget, the games also rewrote Misty's personality in the FRLG and HGSS games to be more in line with her anime characterization as well as gave easter eggs pertaining to her anime characterization (not to mention is the ONLY Gym Leader to have that done in games OTHER than Pokémon Yellow, as not even Brock managed to get that, and he was the only other Gym Leader to join the main cast until Best Wishes), and if I remember correctly, it was the games that have the anime distribute movie Pokémon since Jirachi, not to mention directly influence the plots of the movies based on some elements of the movies plots that are incorporated into various Legendary Pokémon's elements (eg, Soul Dew for Lati@s, Samiya for Manaphy, etc., etc.). There's no reason to break tradition to make a primarily water based region for the first time since Generation III unless they're trying to hint at a return for Misty.
And again, we really can't say that she's going to be part of Ash's team. Just because she's on leaked concept art doesn't mean it's confirmed she'll be joining Ash. Don't forget, one of the openings depicted Jessie with a Weavile, and last I checked, she never got one. Not to mention the teaser poster for DP implied that Paul would be a traveling buddy for Ash alongside Dawn, potentially replacing Brock, yet as soon as DP came out, we discovered that, nope, Brock's back as usual, and Paul, if anything, was Ash's rival.
And don't think I'm saying this just because I'm a big fan of Misty. Even if I utterly hated Misty's guts, I'd still come to that conclusion, because that's the only conclusion that CAN be arrived at, since the only other explanation is pretty much tossed out due to it occurring on the wrong year (remember, the anime started in 1997, NOT 1996, that's the GAMES).
And here's another food of thought for you to ponder. If they didn't intend for Misty to reappear at all, why did the Team Rocket radio show have Ash and Misty not only a special episode dedicated to themselves, but even have Misty and Ash vow to see each other again? I doubt they'd include that last bit if they intended to NOT have her return at some point.
