I'm only speaking on the term of magically changing a nature. to me it would seem as an unfair thing as normal players would more often than not have to hunt for hours, maybe days ( yes, silly but possible ) to find the nature they want. At that, a nature with IVs that please them. If a payer ( excuse me for not knowing a better word ) found a godly IV'd... let's say Dratini, with 29-31 across the board but oh my gosh it's Serious nature! "Oh, I'll just pay to change the nature!" and boom.
It may seem like a mild thing but it does seem slightly unfair to those who aren't avaible to pay.
In simple, I'm just saying this may not be the best of ideas. There's certainly better things that can be done and of course, could be things worse.
Honestly, it's just a matter of my opinion and what I think on the matter. I'm not implying "omg tis 2 bad! DO NOT DO! NO NO NO!!1!", but rather speaking my opinion that this should not be something that should be added. :)