Okay, these are probably the only ones I'd actually pay the writers to make:
Whirl Cup V. 2
Misty returns when Ash and co. are on their second trip through Johto to compete in the Battle Frontier and contests. Ash, Misty, Dawn, Brock, Barry, Wake and Zoey all compete.
Ash: Squirtle, Kingler, Totodile/Croconaw, Corphish, Buizel.
Misty: All pokemon
Brock: Ludicolo and Marshtomp temporarily bought back
Barry: Empoleon
Dawn: Piplup
Zoey: Shellos and Lumineon
Wake: Gyarados, Quagsire and Floatzel
Wake can be seen training with Chuck or something and inform Ash and co. of the Whirl Cup. The group can meet up with Misty as well.
Trainer Cafe Tournement
An epic 7 episode tournement. The slogan can be "Attracting the greatest trainers in the world" to justify having a lot of old characters.
Participants:
Ash
Dawn
Brock
Barry
Paul
Dawn
Zoey
Kenny
Nando
May
Drew
Harley
Solidad
Gary
The tag-along trainers from DP
All 8 gym leaders from Kanto, Johto, Hoeen and Sinnoh
The Elite 4 from each region.
Jessie
James
Harrison, Richie, Morrison, Tyson, and the other league rivals
Jimmy, Marina, Brenden, Lucas, Wally, Silver and the other unseen game characters.
That would be great.