Most important part is to have an actual plot, start to finish. It's embarrassing to see them make a show and not know what they are actually going to do. This is why a shorter season instead of weekly would be better for cohesive storytelling. Instead of trying to hit the deadlines, they could actually make something worth watching.
Using the game material. Almost every single location in the current gen's region should show up, and so should the important characters, ESPECIALLY THE RIVALS. Nowadays, more people are buying the games than watching the anime. Common sense would indicate that promoting popular game characters instead of COTDs that nobody cares about is a good idea, but I guess that's to much to ask for.
Speaking of rivals, the worst thing they could do in my eyes is not use the in game rivals. The games literally hand them on a silver platter, and they still would rather use OC characters. When I saw Rinto (blatant bootleg Wally), I had to stop watching for a couple weeks. Rivals are some of the few characters in this franchise to have actual character development. USE THEM!
The current gen Pokemon should be prioritized. I think it's a great idea to have some previous gen Pokemon on the team (looking at you XY), but Journeys did too much. Lucario, Dragonite, and Gengar overshadowed the two Galar Pokemon. All of those mons have generations of nostalgia behind them. The new Pokemon deserve this chance to shine. Most Galar Pokemon will now be relegated to background characters for the rest of this franchise's existence.
The only fillers should be training/bonding episodes. That's how you turn a "filler" into a episode that contributes to character development. You can't skip these type of episodes without feeling like something is missing because they actually change the structure of the show. Helping a COTD with their personal issues should be a thing of the past.
The Pokemon anime has an infinite amount of options due to how much material the games have accumulated over the years. Every generation, they have access to tons of new characters and Pokemon at their disposal. I know it's too much to ask for, but I wish they would use some common sense and use the material that has been given to them and expanding upon the unsatisfactory parts. The Pokemon anime should be fleshing out what the game provides.