^ if the writers are planning to implement the Togekiss part from Platinum there is a small chance too. Other than that... indeed, the Shinou League, but I think when Ash is back in Pallet Town is more likely.
You mean, if they even intend to implement Togekiss at all in the show. I mean, it's getting less and less likely that Cynthia would use her Platinum Team (I mean, they have not used Byron's Platinum Lineup, nor have they used Candice's Platinum Lineup. If they don't even use Volkner's Platinum Lineup, and instead stick with their DP lineup, then the chances that the Elite 4 or Cynthia using Platinum Lineups (IF Ash is going to even challenge them, that is) are going to be 1% at best.). Plus, going by how the writers have portrayed the Toge line (IE, having it be an uber rare pokemon, even going as far as to treat them as if they were some sort of Legendary Pokemon line, and not giving Togetic a debut in Johto [making it one of only two Generation II Pokemon to not debut in the Johto saga at all [the other being Porygon2]), they definitely won't utilize it as being a random COTD Pokemon or even a POTD.
However, there is still an option that can be considered OTHER than Togekiss, a Sinnoh League Cameo or a Pallet Town reunion to have Misty come back.
That's right, a Whirl Cup-esque competition. I mean, it would prove whether Misty's status as a Gym Leader really does work towards her goal of becoming a WPM or not, and it would give Ash a way to train for either the League or for Volkner. I mean, if they can insert a random Water Pokemon related Pokemon Contest for no reason other than as an excuse for May to appear, they can do the same for Misty.
Now, I'll bet that Misty does worse than before, even though I don't like it, because of how Ash's inexperienced team wins against Gym Leaders. It would also have her realize that doing the Gym simply won't get her closer to becoming a WPM, and if anything, would actually drive her even further away from her goal than before, and she'll decide to retire from becoming a Gym leader, and one of two possible scenarios occur. 1. She returns as a Main Character, or 2. She starts her own journey.