It would be nice to see a tweaking of the algorithms that control the random stores so that characters that are already 7 star are less likely to appear than ones that aren't. but that's probably asking too much.
Heck, I want to see all the toons you have at 7* taken out of the non-random stores (I'm looking at you, GW and Arena).
Since you can spend 400 tokens to get 5 shards for characters you already have at 7*, and then the only use for those shards is for Shard Shop currency where 5 shards = 75 SS currency, as a QoL issue, just remove every character that a player already has at 7*, and as soon as a single character has been removed from that store, add a button to buy 75 SS currency for 400 Tokens.
I once bought a toon on my primary account that I had 7*'d but needed on my 2ndary account. If that toon wasn't there, I couldn't have made that mistake.
Plus there are other toons that I haven't played in a long time, but I have 7*d because they were good during an earlier era or they're good for new players but lose value over time (Lumi). Sometimes I forget that I have a toon at 7* and have to click on the character's picture just to make sure (or I have to click to make sure I'm not thinking of a toon I need on my secondary account while I'm in my primary or vice versa).
If you pull those toons and put in an SS currency button, mistakes are impossible, the shop is less cluttered, and you can make decisions easier & faster.