How about Ash and Misty meeting up for a full-blown, six-on-six water battle? Ash has enough now and it would be a nice change of pace.
Ash could have Kingler, Totodile, Corphish, Buizel, Palpitoad and Froakie/Frogadier/Greninja.
Misty could have Starmie, Corsola, Politoad, Gyarados, Azurill and Horsea (though both now fully evolved.)
Then during training for the battle maybe one or two of Ash's old pokemon could evolve....
But then maybe I should just wake up and accept that this will never happen because, let's face it, the writers of the anime were given the perfect opportunity to bring back Misty, and indeed every other gym leader, in the form of the world tournament. Hell, it was presented to them on a silver platter complete with glowing neon signs that said "WRITE THIS!!!" and instead we got a filler arc on some pointless islands that no one wanted or even liked...