I don’t really remember or care much about stuff from OS, so I’ll only be listing things from AG-onward.
AG: Write the Contests better and show us more of May actually training. Take a few of the very bad Gym Battles and make them....not bad. Rectify Torkoal’s jobber status; give Ash interesting rivals with proper screentime. Not make him lose to Puss in Boots. Many things, I’ll be honest.
DP: Also rectify Torterra’s jobber status. Give more focus to Togekiss and have Dawn’s acquisition of it come earlier. Come up with a better reason for Ambipom to leave than ping-pong. Give Brock something. New characters he can bond with, more captures, something.
BW: As much as I find BW enjoyable, to find it well-written you need to overhaul the whole thing. Make Ash competent, give Iris some more focus, handle his team better, copy-paste the BW game plot and change nothing, don’t introduce random rivals who add nothing to the series and develop the ones already there. That’s a lot.
XY: Make Serena do Sky Battles or the Battle Chateau or even some kind of competitive goal based off of Pokévision. Give Serena a full team and have Pancham & Braixen evolve. The crush isn’t a thing or at the very least it’s not used as bait. Keep her early personality as well as her nicer demeanor. Tone down the gender roles and rigidness. The rivals are not an utter bore. Clemont gets to do more stuff after the Gym Battle. Ash has personality and reacts to things and characters with more variety. That whole Sawyer and Alain rivalry subplot with Greninja was odd to me. I had high expectations for what it’d mean for Ash as well as the other two but in the end it felt a bit...aimless? Idk I just remember not being impressed with the writing (especially after how hyped Alain and Ash’s meeting was). Maybe my own fault though. About Greninja, just give Ash a Mega; Ash-Greninja was so strange. And of course the League, burn it down and write a completely new one.
SM: Tone the silliness down. It was overdone at times and was quite often substituted in place for good writing. I liked Naganadel but replace Melmetal with Passimian or Kommo-o or something else. Have Lillie do stuff and progress actively. Have Mallow and Lana also do stuff and feel like actual characters with actual development. Restructure the entire League. Introduce Skull earlier and just, again, stick to the SM plot. Do not try to redeem or soften Lusamine. Have the school feel and function like an actual school. Let a lot of the more minor subplots develop naturally. Do something -anything!- with the TR and Nanu setup. And while I appreciate the SM art style and what it did for the animation, I’ll be honest. The XY style was the most aesthetically pleasing the show has ever looked, from battles to characters to backgrounds. Doing away with it was a generally big misstep to me.
Journeys: If I was in charge the saga wouldn’t exist. But nevertheless, have Go progress naturally and realistically; both his character and his goal. Actively include Koharu in more episodes and have her tag along much more frequently. Give everyone on Ash’s team proper screentime. Better battle animation and fight choreography. Give plot developments and character reintroductions TIME TO BREATHE with multiple episodes. Bring back the direction of previous series’ and have episodes connect. TR desperately needs more screentime and a capture. No more 2v2’s or 1v1’s please. Slow down the rate at which Ash is climbing the PWC. Have Leon be more relevant; I don’t think the emotional appeal will be there when Ash battles him at this rate. Have Go struggle more with battles and doing things out of his comfort zone. A boatload of revisions for this one.