Something I’d like to see implemented, particularly for 4* characters (but ideally for all characters):
If you have a duplicate unusable cover, in addition to the option of selling it for the listed ISO, be allowed to apply the cover to the character for double the ISO for leveling. For example, if I have a dupe Invisible Woman cover and she is unchamped, I can sell it for 1000 iso or apply it to leveling her specifically at the equivalent of 2000 ISO.
This would likely lead to some of the lower tier 3* and 4* characters receiving levels they otherwise would not have, and could see more of them leveled high enough to see some use, particularly during their boosted weeks. Additionally, this would make the 2* cycle more ISO-friendly as players could simply apply dupe covers to the character to get them rechamped, rather than having to spend other ISO.