You do know that even though he already has the badges, it doesn't stop him from re-challenging the Gym's(like you can do in later generations of games). I mean in SM43 he had a battle with Misty(even if it was "unofficial") and during Season 1, Ash made "promises" to return. Wouldn't it be a change of pace for him to actually follow through on such promises like revisiting Pewter(battling Forrest), Cerulean(maybe more than just a 1v1 with Misty... heck he could battle her sister Daisy), Celadon(a fire kind of interrupted his battle with Erika), Saffron(Haunter and an unresolved Psychic showdown anyone?) and Fuchsia(is Janine leading it now?). Plus we have no freaking idea who the VIRIDIAN GYM LEADER is. Agatha was the hold-over until the new leader arrived so that alone is justification for doing it.
Also if you don't like the idea of Ash going through Kanto again, you could have him accompanying a Female protagonist(like Serena or "Leaf") in doing such a task themselves. Plus there's Pidgeot, Primeape and all these Generation 1 Pokemon Ash needs to reunite with(I'd love to see Butterfree again).