Honestly not sure if i would support this because, for example this shiny Skarmory worth 5m aprox, one with sturdy a guy got offered 10m, and one with sturdy + impish a guy got offered 20m+, so if someone get a "Bad" high tier shiny, they can easy change them and make them expensive or much better, so i don't totally support this, on legendaries is different cause they are untradeable, but on shinys they will ruin the good ones, will make them cheaper...
Sorry main language is spanish..