(I know this a bold thing to ask since we can't even check IVs in-game yet, but bear with me.)
Getting bad IVs sucks, especially when a unit's kit is designed to work with a particular stat (e.g. Ayra, the Myrrhs). Some would say that IS would never let us fix bad stats because they'd lose sales from whales who would pull on a banner for an "ideal" unit, and I agree that that's probably the reason they haven't done so already. However IS has already made some QoL that risked their bottom line, with the most recent example being Combat Manuals (since buying more space is now pointless). So, if IV-fixing is ever made real, how would you expect it to be implemented?
Right now we have two currencies that are completely useless (whether IS wants to admit it or not): Arena Medals and Light's Blessings. Why not let us use one of these to adjust a unit's IVs? And if they chose the Medals, it'd be another incentive to play Arena for those who've turned away from it. And if getting to choose our exact IVs is too much to ask, letting us randomise the IVs of a specific unit could work too, as long as you lowered the cost to compensate - this way people would only reroll for units who have the worst IVs possible LIKE MY -SPD/+RES LAEGJARN and be more likely to just cross their fingers for a lucky summon while pulling for other units (e.g. if someone wants a 4* unit for merging).
Thoughts?
P.S. I hope this question is open enough to stand on its own, @eclipse?