I was just thinking about this a few days ago and it's a big reason why I keep going back to showdown instead of playing pro.
My idea was: the more rerolls you put into a specific legendary, the higher the iv floor is for that legend. With a cap ofc.
For example:
1st rr: iv floor of 1
5th rr: iv floor of 5
10th rr: iv floor of 10.
Numbers can be tweaked and stop wherever staff sees fit.
As for hidden power, just make an expensive item that lets us pick it and it and adjusts our legends current ivs up or down by 1 to fit that.