I don't recall when did I get the desire for a more serious/mature anime, but I can tell that Pokémon Chronicles and the "recent" B2/W2 promotional trailer helped very much.
Assuming that this new one would air along with the current anime, I would like to see new main characters that traveled for at least two regions because it would be boring to always see a rookie, unless the anime started with a seasoned trainer.
I simply can't no longer identify myself with Ash. The only thing he has achieved was that Orange League Trophy and a "herd" of Pokémon that must be expensive for Oak to keep. And speaking of Oaks, look at Gary: he was an obnoxious kid that used to drive a car with a whole bunch of mindless cheerleaders but has since turned into a doctor/scientist/researcher/whatever at some place in Hoenn and (if I recall it correctly) studying a very interesting subject - fossils. Ash? Still trying to be a champion but will most likely end as a hobo. Oh, and talk about stealing your thunder: Ash is getting greedy when it comes to success because when there's some event that is aimed at one of his companions, if he can particpate then he'll do it, no matter what. A contest? I'm in. An aerial competition? Count me in. A Dragon-type tournament? Oh boy, let me just call my Pokémon buttler Oak and have him send my Charizard.
Another thing : COOKING! Seriously, Ash can't live without a gourmet cooking slave. Unlike 99,9% of trainers, Ash relies on a guy that when someone gets hungry, pops out of his bag a whole set of table, seats, ingredients and pots. Good thing they replaced Brock because that women/Croagunk gag was pissing me off. While Cilan keeps the cooking cliché, at least he has all those unexpected hobbies and the "evaluation time" isn't as frequent as to become too much. Still, at the end of the day: no cooking guy, the main character travels alone.
Rivals - Despite hating Paul because of his methods I must say that as a rival he was almost, if not, perfect. He showed us what we see and (some) do in competitive battling: the search of that great Pokémon, even if that means leaving weak ones behind and playing eugenics with Pokémon. But, to the much surprised lil' Ash that was a new one in his perfect Pokémon world (T.Rocket and poachers don't ring a bell!) and with Paul we see the old tailing thing of Gary, in this case, regarding the Pokémon both Paul and Ash trained. Really, did Ash needed to have almost every single Pokémon that Paul had? And the story repeated itself with Trip with Snivy and Pidove. One more thing lil'Ash seems to forget: there's no such thing as trademarking moves. Ash likes to follow the steps of the E4 he meets and to adopt some quirks from trainers but when Paul decides to mimick that "counter-defence" or something, he gets shocked. Well, call officer Jenny.
I really feel sorry for Hunter J. because, while not being a rival per se, she was one hell of a character and I wished that her Drapion would've crushed Ash and sent him to the afterlife!
Evil team/Team Rocket: no more silly trio. I liked the beggining of Best Wishes because James, Jessie and Meowth decided to go on a serious and professional tone but then...we all know how that's going.
One thing that keeps pissing me is how Ash learns of an evil team that seeks something big, some episodes latter he twarts their plans while getting to meet some legendaries and the next episode he simple goes to his own life of challeging the next gym...Talk about life-changing events, for Ash they as trivial as...as....doing whatever he does.
Breaking promises: by this time, how many times has Ash promised to have a rematch with trainers? And how much of those have come to happen? Btw, poor Pidgeot is still waiting for him. I've got an ideia for an episode of this more serious anime: have the protagonist meet a lonely Pidgeot that because of having being abandoned, is attacking all trainers that pass through Viridian (or Route1?). Then, he'll battle it and capture it (I don't remember if Ash did release it or just didn't never called it back into it's PKball).
Battles: no more last minute miracle strength, no more "wins, wins, loses, learns a lesson, go into hard training, wins rematch". Trainers should be more careful when deciding to battle or not, while battling and when risking their Pokémon lives. Being a trainer ain't easy (except for Ash with that cooking slave and brainwashed Pokémon) and I want to see that in the anime. Oh, and like some have already said here before, make the Pokémon dodge attacks on it's own and not only when ordered to.
Cries: I think that having the Pokémon saying their name instead of having actual cries may be because it's easier for the dubbing, instead of having to create cries with different tones. Still, this is something that can be discussed.
PS: my favorite episodes are the Peak Maiden one and the Ninetales/Brock doppelganger. Those where sad episodes but that is what makes them good and unforgettable. I wish there were a few more like them.