I like how when you learn a move in Legends Arceus, you could use it however you please. My idea is to have a button on the moves page that would let you view any move currently learned. Selecting an old move would have a similar effect to if you JUST got the move, where you can select what move it would replace.
The idea is fairly simple, which is to add Belts that give the effects of the move. The Rapid Band, for instance, lowers power, makes all moves lose 2pp, BUT there's a 30% to do the move twice in a row, but the second one won't cost PP, as a way to implement how "Action Speed" was changed by Agile, potentially giving a Pokemon the ability to do 2 attacks. The Focused Band, on the other hand, boosts both forms of Attack, but your speed is lowered by holding it.
Here are some other ideas that are more random...
In a battle, have the ability to view the type and stat changes for both the user and foe, along with, maybe, the moves that the foe has used. This was a useful feature in Sw+Sh
Anoher Gen 8 Feature, but, in battle, you'll be given a symbol if a move was Super Effective, Not Very Effective, or had No Effect whatsoever
What's that, buddy?
When your Pokemon is currently out and has a bond with you, they would stick close to you, following you. When there's a hidden item, it would make its cry and scurry off to it.
Power of Friendship
Give the Pokemon advantage when at high friendships, like the recent games, where they could endure normally fatal attacks or shrug off effects like poison.
Simply give the trainer the ability to switch a Pokemon's ability, such as switching a Zigzagoon's Gluttony to Pickup, but I would recommend excluding hidden abilities from the mix. Speaking of which...
Here are some ideas for more Charms, for instance ones that increase spawns for bosses, more rare Pokemon, or even things like having Egg Moves or Hidden Abilities.
Need I say more? Bags where you can store battle items that can be used in battle should be craftable.
Keep them Distracted
This is somewhat odd, but have you be able to increase the spawns of Pokemon in a certain Egg group by having a Pokemon of that group out (as if them being there makes the others calm enough to show up
Multiplayer Related Stuff
This one is more unusual, but useful for Multiplayer. If you use a Mail, you could sign it and select a player online. Give it to a flying type and they'll leave for a period of time as they warp to the recipient, who would just have to right click them to get the mail.
Simply let players use a device to become part of a Party so they could be able to be in 2v2 battles.