First how about in addition to Ash they are 7-16 other trainers. The trainers are rivals or friends to Ash but, they also have their own relationships. The would go together in there own groups change groups through drama. Not all episodes would have Ash thus bringing variety to filler. There would be so much going on that nothing would get stale. I love this Idea even if it is just for one series.
Second, Ash could be constantly thinking of ways to achieve his goals. Have a three part saga where he trains somewhere. Have him go to battling circles to set up a filler stories with particular trainers but further his goal in the short term. Have him seek out the skills and people that make him a better trainer instead of him bumping into them.
Third, introduce new villains and focus on game villains more.
Well I only intensely like the first one but sure at least one of the writers can think of something better than what I said or filler.