I would greatly have preferred if they had been somewhat realistic in the pokémon anime and changed protagonists in every region. Like the Pokémon Adventures manga did! The older characters still reappear now and then, but they actually age and learn new things or grow into new professions and trainer styles. Sure, some former companions in the current pokémon anime have "grown" and been shown to be successful in contests in other regions and things like that, but they don't actually ever grow.
Having Ash & Pikachu being constant and never ageing or changing very much is a huge key point in why I find it hard to truly enjoy the anime. It's just... not even half realistic. Sure, realism is a strange word to use in the context of pokémon, haha. But even Digimon showed their characters aging. Why, oh why, did they let Ash leave his team behind and travel to Hoenn to "start anew" once upon a time? Before that, all was well, really.
That being said, I still enjoy the anime enough as it is, I guess d: it's cute. But mostly for the pokémon, less so for the characters, sadly.