1. Bring back Misty and Brock.
2. Have everyone age.
3. Have Ash and his team gain experience. I know they like to reset Ash's age and the levels of his Pokémon each season (since the Hoenn season), but I still believe there can be many challenges even if all of Ash's Pokémon were of high levels.
4. Bring back the classic art style. Or at the very least have it like it is in I Choose You/The Power of Us.
5. Have Ash compete in the Indigo League again. I feel it's unfinished business, he only lost against Richie because Charizard refused to listen.
1. Agreed, and after the good job they've done for Brock, I actually would want him back (had you asked me before the Kanto Reunion arc, I would not have wanted him back after the crap they did from making him not focus on his breeder goal or doing much of anything other than cooking and cleaning, and in the case of DP also not even trying to rejoin Ash, to him being turned into a womanizer). And I certainly want Misty back, especially after the writers did a massive disservice to her by throwing her into a gym despite her goal of Water Pokémon Master requiring she reach at the barest minimum the Elite 4 level (to say little about how before XY or especially the Kanto Reunion arc in SM, the Gym leaders were depicted as being JJM-level joke characters starting with AG). Even if she doesn't return, at the very least, she should be allowed to actually resume her goal of WPM by traveling.
2. Pretty much agreed there. It's one thing if this were like, say, the Simpsons where there's negative continuity abound and care more about a joke a day, but Pokémon's more like the Rocky movies, which DO ultimately require aging regarding the nature of their goals. And besides, even the writers commentary for the Orange Islands arc indicated Ash aged with the episode where Pidgeotto evolved into Pidgeot.
3. Also agreed there as well. Especially when, if I must be honest, the only saga where there's actual discernable growth was in Johto. DP and XY did have him getting better, sure, and even AG at least had the Battle Frontier to make up for the poor results of Hoenn, but there's little to no continuity there (heck, you could skip AG and not miss much).
4. Yeah, have to agree there. Didn't mind Misty and Brock's redesigns too much (they still pale in comparison to their original depictions, but at least they're tolerable compared to Ash's redesign), but it's better off if we get the original designs, at the very least the M20/21 designs.
5. Well, it certainly would make things full circle, I'll give you that. Not sure if that will happen, though, since they're either going to promote the new games, or just cut it short.