IV-boosting items

The danger to this is that the expectation will be a perfect lineup. Fighting Latios or Latias? You don't have 6 perfect maxed level Rayquazas? Then you're doing it wrong.

The game remains interesting because people have to find different solutions to the problems presented. Want to solo Machamp? Mewtwo is good. Don't have that, try Espeon, or Double Flying Rayquaza, or Lugia, or Ho-Oh. We all have choices, with different costs, different strengths and weaknesses (Lugia crushes fighting Machamp, but isn't great vs. double Steel Machamp, etc.)

So there would have to be curbs and limits on things. Finding a pokemon with 15 attack, 15 defense, and 6 stamina shouldn't set one off on a quest to jack that stamina up 9 points. But it's doable, and it would be nice to be able to buff up 98% or 96% pokemon to 100%. Just not too much, and it should cost a lot.

If such a feature were to happen, I'd envision the cost (stardust, candy, or whatever) to be scaled depending on the current IVs, pretty much the same as how levelling works. So, powering up from 0% would be very cheap, but going from 98% to 100% would be very expensive.

I would also love for battling (perhaps also including gym defence) to be a requirement.

I'm not against the idea, but I think the cost scheme should be different. It should cost more the closer you are to perfection so that going from 96% to 98% and to 100% should be astronomical, but going from 80% to 82% not so much.
I for example don't raid alot, but still have 9 legendaries at 96% or higher. With your suggestion it will take me 2-3 months to bring them to 100% and that's too easy.

I've always been an advocate of using the Pokémon in order to raise up the Stats rather than buying something or using an item.

For example: Want to raise your Attack? Attack Raid Bosses or Defeat Gym Defenders. How this would work in my head would be like this: You need to defeat (10x Next IV Level) Gym Defenders or Raid Bosses to raise the Attack IV by 1. Say you have a Tyranitar with Attack 14 - you would need to defeat 150 Gym Defenders or Raid Bosses with that Tyranitar. Which sounds simple enough, but you're looking at needing to take down 8 gyms with 6 Pokémon in each at Full Motivation (+6 other Raid Bosses or Gym Defenders) with that single Tyranitar in order to raise the Attack to 15. Which is a heck of a challenge, even for a hardcore player on the grind.

For Defense, it would be (10x Next Level) Gym Defenses and Dodges, where every time your Pokémon was in a Gym Battle as a Defender it got 1 point - and got 5 points if it actually knocked an Attacker out. And in a Raid Battle or Gym Battle, you could get 1 Point per battle if you dodged a Charged Move. (not 1 point per Dodge, but 1 point per Battle.)

For Stamina, I'd tie it to a combination of Gym Defense and Attacking; it takes (10x Next Level) to raise the Stamina, and you get 1 point every time you Attack a Gym Defender/Raid Boss or Defend a Gym.

Not a perfect system by any means, and probably too simplistic to work. Also likely able to be exploited - using the Tyranitar example, it would be as fast to take a 14/14/14 Ttar to 15/15/15 as it would a 14/15/15 to 15/15/15 as all 3 stats could be raised at the same time. (Which I personally don't have a problem with, but could be seen as exploitative.) The point isn't that I've got the right system for how to do it, but that I'd prefer to have a system that would encourage me to actually "Train" my Pokémon rather than use a item to simply strengthen it.

With the new update, they have an items tab when you have a Pokémon pulled up, next to transfer and favorite options. This will make it easier when using IV boosting items. And I don’t think there should be a requirement to use them. When you level up a Pokémon to level 40 that’s a 96-98%, then catch a better IV one, it’s like all those hundreds of thousands of stardust went to waste. I think maybe having the Pokémon maxed out for your level may be a requirement, but doing anything extra after spending hundreds of candy and over a quarter a million stardust would not be something that would make sense.

I'm gonna preface this by stating I think IV boosting is a terrible idea for PoGo - as the first comment stated it would set a terrible precedent as raid partners would begin to expect nothing less than perfection, especially the more hardcore ones. Also IVs are incredibly over-emphasized with attack being the only one worth caring about due to raid breakpoints.

Having said that, this exists in the main series in the form of bottlecaps, which let you correct faulty IVs on a Pokemon - the only catch is these caps are a pain to get and can ONLY be used on level 100 Pokemon. If Niantic were to implement an equivalent to this I would expect similar requirements to be met - maybe the item is only given away via skill-intensive research and it can ONLY be used on fully maxed Pokemon.

Another comment - there are a cadre of players whose prime goal in the game is to get a full pokedex of perfect pokemon. One guy in my area has 37 perfect pokemon powered up to level 40. (Me, I have three powered up to 40, and they are 98%, 96%, and 98%, and I think that's pretty solid.)

But if we allow IV bumping, the people who worked their asses off for perfects, who would hear through facebook or discord that there's a perfect level 6 chinchou at such and such a place and dashed over there hoping to get it, will be really pissed that any duffer can get perfects with a little work.

Now, that's not my game, I want effective pokemon for raids, gym battling, and gym defense, and a 96% to me is just two tiny steps below a 100%. But for people who that's their game, it will destroy the game for them.

There are also collectors (of 82% and up, the living wonder pokedex crowd) or of any pokemon. Eventually, if/when breeding becomes a thing, there will be breeders too, wanting to breed pokemon for fun. Trading, I won't even go there.

But as they add new mechanics, the game evolves and changes. Remember back in July of 2016, when it was cool to go out and catch a Rattata or a Pidgey, and spinning stops was exciting? That was all we had, and now there's so much more.

Anyway, I could see IV buffing as a thing, but it would have to be really hard, really limited, and highly resistant to abuse, which will require some very clever thinking by the coders and gamers at Niantic.

I imagine getting various items that do things like:
-reroll a specific IV
-reroll a specific IV from 10-15 (ie, hatch values)
-reroll all IVs from 10-15
-add 1 point to attack
-add 1 point to defense
-add 1 point to stamina
-add 1 point to a random IV (0 added of IV already max)