Many have asked for this… I’m still not sure how it would work on the long term… I mean I guess each new upgrade makes extractions cost more ? 
I think legendaries already have some chance to get better/higher rolls… They’re dropping quite a bit in the game, especially at mid-high-T4 levels. The problem is having a good legendary with the aspect what you need is totally abysmal
This can be simply fixed by adding an option to remove the inherent affix from a legendary but keep the item, so you can imprint another on it’s spot 