1: First, and most importantly, switching Pokemon should not have priority. It should be based on your Pokemon's speed, like a move, so faster Pokemon can be switched out like normal, but priority moves can hit them like Pursuit currently does, while slower Pokemon would have to take a hit first.
2: Make held items visible, as though everything on the battlefield had Frisk. In-universe, they should be obviously visible unless the Pokemon in question has a place to hide them, and it'd be more fun to know which Pokemon had Choice Items, for example. There can be an Ability that hides them again, of course.
3: No in-battle healing items, by either the player or their opponents. Nothing's more annoying than a Gym Leader healing their ace back to full health, especially if that places the player in a position where they have to do so as well to win the battle.
4: Gym Leaders and similar boss battles should start with parties of three, and add one more Pokemon per badge until they get a full team at the fourth gym.
5: Add difficulty settings. It's baffling to me that they were only done (and made nigh-inaccessible) in B2W2 and then forgotten.
6: Make EVs into some sort of manual point allocation system rather than the tedious grinding we have now.
7: Remove Evasiveness as a stat altogether; Minimize and Double Team should instead decrease the accuracy of all opponent Pokemon, and Defog and Sweet Scent should raise the party's accuracy.
8: Boost the accuracy of most physical Rock moves. Their inaccuracy is the primary reason that Rock is my least favorite type.
1. Reduce the stat changes from buffs/debuffs. Might go with diminishing returns here: 50% for each of the first two stages (unchanged), 25% for each of the next two stages, and 12.5% for each of the last two stages. Adds up to a x2.75 stat multiplier at +6, down from a x4 stat multiplier. Goes similar for debuffs, resulting in 4/11 of the usual at -6.
2. Remove some move coverage from most Pokemon. Does not want to return to the days of Pokemon learning almost nothing. Example: Looked at Gholdengo. Why does it learn Sandstorm, Charge Beam, Thunder Punch, Power Gem, Psychic, Focus Blast, and a few others? (Kind of understands Dazzling Gleam, given the coins, although not the type of the move.) Leave the wider move pool to weaker Pokemon.
1. I want to bring something sophisticated. That is including the dodge mechanism, as it is shown in anime series. For this to happen, there should be something like "Dodge" factor in stats of Pokemon, whose maximum being capped at 50% rate. Pokemon with higher speeds tend to have faster growth rate in this sector, and vice-versa for accordingly. Although move accuracy exists, but that's from opponent's(or ours) attack perspective, and it can't be controlled as such(excluding moves like Double Team, Minimize, etc). So having "Dodge" brings some new flavor into the battles. In order to it not being redundant and too much reliant, there should be something like Dodge can only be used after 3 or 4 turn, with first Dodge being available to all.
I disagree with all of these.
Buffs/debuffs are fine as-is; Pokemon battles are fast-paced enough that using a turn on a statboost move is a serious risk against an equal opponent. If you have the opportunity to max out a stat, and it isn't from Speed Boost or Belly Drum, either your opponent screwed up somewhere and they deserve to get their team swept, or you cheesed the match with healing items.
The current rate of move coverage is good; it allows Pokemon to avoid being pigeonholed into certain movesets. (e.g. Staraptor normally always runs some combination of Brave Bird, Double Edge, Close Combat, and U-Turn.) It also allows players to focus more on their favorite move types even when not running those types of Pokemon; I often run Shadow Ball/Shadow Claw on non-Ghost Pokemon, for example.
Inaccuracy and Evasiveness are a nightmare to deal with already, and we have Protect and Double Team as near-universal TM moves if you really want to evade stuff. It does not need to be a separate mechanic.