I'm going to post the obvious one here:
Ash actually wins a Pokemon League.
This would dramatically improve his character, because firstly it would make the entire show, and his journey up to that point, mean something. The Pokemon anime has been one giant game of "your princess is in another castle" up to this point with Ash's repeated losses of any significant tournament: sorry Ash, go to the next region and try again. It makes all of his character development, and all of his Pokemon's character development, utterly meaningless. To have him actually win, to be rewarded for travelling through the region, would make everything so much better. That it'd improve his Pokemon's characters as well would be a great bonus. Remember how amazing it was when Ash's Infernape overcame Paul's Electivire? It was the culmination of an entire arc's worth of rivalry, bitter failures, and emotional scarring. Wouldn't it be nice to have that for Ash and for all of his Pokemon? Just once?
There is nothing to say that this would be the end either, or that Ash couldn't continue to improve following this victory. After he won the Battle Frontier, he was utterly curbstomped by Gary, which prompted the Sinnoh seasons, which were the best the anime has ever had to offer in my opinion. Not having Ash reset to factory default would be another great way to improve his character, for obvious reasons. He didn't lose all knowledge of his previous journey in Johto, or when going from Battle Frontier to Sinnoh. He's not a bloody goldfish. To see him actually develop as a person outside of a single series would be great.
Also, if Brock comes back, let's call in his hormones just a tad. It was funny when it was just Nurse Joy and Officer Jenny, especially when it was lampshaded by Brock himself, but EVERY girl older than he is? Got very tiresome, very quickly.