Ash did win a league. Granted it was the Orange Island leagues and has pretty much no impact anywhere else in the anime to date, but Ash did win a league.
Yeah, it would be kind of nice to see Ash win the Kalos league itself only to have his match, technically a rematch I suppose, against Diantha only to lose. It would put him further up in the standings than he has ever been before. All of those things mentioned "unknown regions, exploration of new kinds of pokemon, environments, legendaries, myths, small culture things" do happen in the anime but on smaller scales and most fans end up calling those filler episodes, and are done without forgoing the idea of gym battles and the league. I do think this could be done more than it is though.
I understand that there has to be something after being champion, but maybe Ash, or anyone in that position for that matter, should just concentrate on climbing the hill first then worry about the mountain. Alder can worry about the proverbial mountain because he is already at the top of that hill.
Character development...what exactly do you mean by this? One could say that showing how one (over)reacts in gym battles is character development though as Ash proved with his match against Elesa it doesn't always mean it's good development. The kind of Pokémon one catches could be considered character development, like when Ash caught the Squirtle he really wanted to start out with and Brock getting Happiny (later Chansey) was a factor that lead him to decide to be a Pokémon doctor instead of a Pokémon Breeder. Dawn lost like three straight Pokémon contests and had to alter her strategies to finally start winning, does that count? I mean, if Serena fails her first showcase but then learns something to help her win the next one is that character development? Towards the start of XY Ash lost to Viola the first time, trained to prepare for her moves, had a small conversation with Serena that lead to a lightbulb moment, and then challenged her again for the badge and won, is that character development?
How much development can a character go through when someone has watched that character for over a decade? I agree that there is room for some improvement in character development but what is it that is being looked for here, especially on a children's show?
The problem with making someone a child can idolize (and let's be honest some would say it's sad that a child has to look to a TV character to find this, but that's another topic entirely) is that people look for different things. They can't make everyone happy. I wouldn't have Ash as an idol even as 8-10 year old but then again I can't think of any kids TV shows that I watched that has a character I would've idolized.