Hey there,
I think your idea is not bad. However, nature has a huge impact so I would say 50-50% (maybe 60-40% actually depends on your preferences).
There are a lot of bad natures for pokemon, but it needs more attention in the program than +1 1 or -1. Because you can have a nature that favors one special ability of your pokemon, and others favor 2 abilities (for example: Gengar: +SPD & -ATK) The reason this favors gengar on 2 things is because his normal ATK doesn't matter that much. So other things won't get worse.
This reminded me a lot of breeding. If the program was meant for breeding, keep in mind that this will not come in PRO.
Reference to Sinnay his answer on breeding idea post: https://pokemonrevolution.net/forum/viewtopic.php?f=7&t=8650&p=53687#p53687