Most of my ideas are firmly in the 'I have no idea how tough or easy this would be to program but damn it'd be cool if-' category so. Hope they're interesting? XD
The player character/trainer having something like a limit break gauge to do stuff in battles to support their pokemon. Something like 'you start with 3 points out of a gauge of 10 or something, you get a point for each mon (yours or the opponents) that is KOed and you have a list of things you can use those points on' like: Without using up your pokemon's turn you can set up a field effect or weather, raise a stat, heal a bit, inflict a status aliment on the opponent, etc with bigger/more powerful the effect the more points it takes (might also be cool to sync up Mega evolving, terastalization, and Z moves with that system maybe but keeping those limited to 'one use per battle each' is still pretty valid)
Mostly thinking this would be a great way to make the human character feel like they're actually actively helping their pokemon as opposed to just being there barring item use and could be a really distinctive way to make strong opposing trainers feel unique in a game, akin Reborn's field system but less location based and more 'an active thing the trainers be doing' so like, lorewise it's like that cowboy character used their gauge to shoot a gun to deal chip damage against mons relying on study or shoots a tranq to inflict status, whereas a psychic sets up psychic terrain or use telekinesis on a mon, and a doctor might heal or buff their mons, etc).
The ability to make any single mon 'evolve' (for lack of a better term) into the player's signature mon that grows in power over the course of the game. IDK how to word this idea properly honestly but was thinking that mechanics wise it'd be something like 'the signature pokemon of choice evolves into a set aside template that gets stronger as the story goes on with use of this one item, and this template takes the sprite of the original mon'.
Not sure how to handle the typing or the move pool, probably would be easiest if the 'signature mon' just had a damn enormous movepool and became a custom 'neutral to everything' typing (aka the only thing that it'd actually get from the original mon would be the name, personality, evs, ivs and the sprite') and its probably a hell of a lot of work for just allowing a player to make their Bellsprout able to murder even god mons without forcing the use of specific items or strategies but IDK I just thought something like that would be really neat for vibes~ XD
A very niche one since I doubt anyone cares about this but me but: A 'personality and quirks' page on the Pokemon summary that's not linked to stats or how good/bad the Pokemon is. Natures and the stat 'descriptors' are alright-ish for characterising a mon but... I just really want to be able to have a gentle Fearow or a mon that likes spicy stuff and/or sleeping a lot, without it forcing me to worry about whether or not it'll be a subpar example of it's species, you know? (*also changes the 'natures' into 'attack styles' instead for the same thing).
Speed or Attack equivalent of Evolvite, purely for the fun/chaos value of them potentially existing.
(And adjusting all the 'evovite' items to adjust for unevolving mons with very low stat totals would also be pretty cool but probably a bit more difficult than the above to do).
A special move tutor that can teach any mon a specific signature move based off IDK some kind of hidden individual value every mon has (not sure if actual IVs would suffice or if it'd be easier to just make every mon roll a number between 1 and however many moves are in the game) but yeah. Something like 'oh wow my Glameow's special fancy signature move is Land's Wrath but ugh my torkoal only gets splash' or something).
Not important but just a bit of fun to make every mon a little bit more unique~
(Actually if the 'insert new value roulette' thing works you could have more than one of these tutors to add even more rolls of the dice. Just a thought though XD)
A fakemon that evolves based off of being shiny. (And if you wanna still have two different kinds of sprites for it, just add an item that allows a mon to switch between appearing shiny or not -just, non shinys with the paint don't have the star).
Have a fishing like mechanic for land, like 'digging' in rocky areas or 'trapping' in forests/tall grass for more mon location variety. ...'Scrying' for ghosts or psychic types could also be fun~ XD
A space/scifi themed evil team that uses mons like the Clefairy line, the Elgyem line, Deoxys, ultra beasts, future paradox pokemon and the like. C'mon please..! It'd be tough and incredibly awesome, and you could spice it up with 'kidnapped Miltank' or something periodically~ XD
Hidden away pokemon villages/mini towns (tucked away kinda like Hidden Grottos in gen 5) where the player can find little communities of pokemon that don't battle or fight you (aside maybe a few optional ones) but might communicate with you a bit, give you items or quests, run little shops where berries are currency, etc. I just want me some 'PMD games could totally exist alongside a world with humans' vibes ok? XD
Places like a bussiness or town where ingame money can be invested to upgrade them; it's an RPG classic but I don't think I've seen it done in many Pokemon game so yeah. More of that please.
A place which does berry farming for me. I like caring for berries too but I'm awful at it, and. You could also combine this or the above with something like Sword and Shield's Pokemon jobs system or what Sun and Moon did with Poke Pelago, for helping the player feel like their PC mons aren't just rotting away in the PC forever or something.
And that's all for now. Hope some of these were interesting, even if probably a bit too much work to think of bothering with XD