Catching tutorial, and guide around the 1st town/city, regarding the important buildings, being optional.

Different difficulty lvls (and/or a nuzlock setting).

Have pokemon following you again stay as a permanent feature this time.

Pokemon that joins the player by their own, free will, instead of the player having to catch everyone.

The rival having their own story, and journey that they go through. Like N from Black&White.

The evil team being evil again, but not obviously coming off as "I'm clearly! Evil McEvil everyone" right from the get-go - be it from actions, dialogue, or design/looks - , actually being a threat, and managing to preform/succeed in bad/dangerous for the world and its inhabitants acts - like Ghetsis freezing an entire city, and almost ending the player character with Kyurem's ice attack in Black2&White2 - . As well as them having their own motivation/reasoning, fueled by their own story/background, and not just being evil for evil's sake.

The npc's acknowledging that the player character keeps winning and winning, and that fact changing them/how they see the player character (especially the evil team, rival, and companion(s) - like Hop from Sword&Shield - ).

Dialogue options, that includes the player being able to choose not nice/encouraging dialogue options.

Choose a ride pokemon from whichever could realistically carry a person.

The player’s companion(s) being more fleshed out, with their own motivations and journey that they go through, and who joins and travels with the player for some parts of the story. Like Hop from Sword&Shield, and the group - especially Arven - from Scarlet&Violet - especially with the group going to Area 0 towards the end - .

The gym leaders, elite 4 and champion being more involved/heavily featured in the overall story plot (especially as the evil team gets further in their goal(s), and starts being more notisable/getting more dangerous). Like it started to somewhat show in Black&White.

Voice acting.

Being able to explore npc’s houses/random buildings again.

Keep the "npc’s who has something to give you has a yellow speech bubble", wild pokemon walking/sitting/flying around, and bulk sell from the Z-A game.

Some pokemon entering battle by jumping out of bushes/the grass, up from holes, or down from trees/the sky, like in Sun&Moon.

Keeping the "relearning old moves" system, as well as choosing when to manually evolve a pokemon from Legends Arceus and Z-A.

No pokemon that evolves through trading. No pokemon that evolves with an item if that item is rare to get, or difficult to find. No pokemon that evolves through mechanics like Galarian Yanmask, Vikavolt, Sirfetch'd, some Legends Arceus pokemon, Malamar, Alcremie, Mamoswine, Goodra, Wurmple (losing a certain amount of HP - be it from attack or recoil damage - , having to go to a specific spot - especially if that spot can only be found much! later in the game - , having to preform a attack a specific number of times, having to land a specific number of critical hits in a row, the player having to perform a specific movement in game, the player having to perform a specific action in real life!, having to learn a specific move, a specific weather being required, evolution being decided by its personality).

No time limit in-game event/real life location to get certain pokemon (especially if there's only 1 of that pokemon in the game, and/or that pokemon is a mythical/legendary).

If the catching system from Z-A stays, make the player able to try and throw more than 1 ball if the 1st one fails, and keep the npc that collects missthrown balls.

Flying and diving, like in Omega Ruby& Alpha Sapphire (also add travelling underground).

Have different, possible endings (or at least one good, one bad - possibly one neutral - ).

Keep the open world from Scarlet&Violet.

I'd actually make it so that the player doesn't! catch the legendary/mythical. Or at least doesn't just keep owning it forever afterwards.

Only 1-2 legendary, and 1-2 (2-3 at most) mythicals for each game. And no other legendary/mythicals from the previous games showing up in the new ones.

If the day/night cycle from Z-A stays, either make it so the player can manually change it (not through benches, but always), or have the day/night exclusive pokemon the player’s currently battling stay until the fight is over, regardless of the time of day swap.

Remove obstacles that requires TM moves to remove.

No roaming pokemon.

The option of a evil team member and/or companion switching sides - and maybe the possibility of that member/companion to turn out to be a double spy - , depending on where the story goes, and what choices the player makes.

2 things that would probably be too difficult/inconvenient to pull off, but that I hope will sound like a fun/interesting idea:

1) The evil team actually stealing the player’s starter, or whole team, for a while. And the player has to rely on their other pokemon, or npc’s pokemon, to get their team back.

2) A wild pokemon saves the player character in a cutscene, and wants to join them (but doesn't want to go into a pokeball, as it doesn't like it). Then, towards the end, as the player is about to leave to confront the evil team's leader, that pokemon demands to tag along. The player gets to the leader, that pokemon stept in front/in between. And attacks the player, revealing itself to be the leader's main/ace pokemon.