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Having "UX fixes for various sites" is too generic and varied in description - it would be a hard sell.

However, I could see it working if you were to drill down to one specific feature, like hiding the login prompt for a selection of sites.

Adblock can fix arbitrary sites to a certain extent (only through removing elements, but it could have accomplished what this extension does). Select an element to hide, and if enough people do then it gets added to the blocklist.

Make a list of the worst offenders (it would have to be done per site) and a userscript could easily be made for them.