i think it would be cool to have a coin item or a reward item that is obtained in a long quest that changes a pokemon nature that you want changed. like if you catch a pokemon that you spent a long time hunting for but comes with the wrong nature but good ivs, you would be able to keep it and change nature to what you want(or have it change to a random nature)
for coin id think like 50c would be a good price or 100c if you can pick nature.
or have an quest that makes you catch 50-100 pokemon of the same pokemon you want changed. like for gyarados you would have to catch 100 karps and get a item that lets you change any gyarados nature to what you want(so you can make it jolly/adament).but would be like that for every pokemon
some times you get a perfect iv pokemon but nature is garbage, this item could be used. not everyone wants to spend a lot of money buying a pokemon because they cant catch the one they want
(just a thought dont kill me over it xD)