This is just a thought I had, not sure if anyone else likes it... I think it would be useful to have the ability for each user (not global) to optionally customize the background color of tags they are interested in on Stack Exchange websites. Currently users can mark preferred tags/ignored tags, but this is a somewhat limited filtering mechanism in that it doesn't allow you to distinguish specific tags amongst the rest.
This is a more of an aesthetics enhancement than anything because obviously users already have the ability to filter by a specific tag. Being able to assign colors to different tags would merely allow users a slightly more custom experience when viewing the lists of questions. Obviously, we already have certain custom formatted tags (primarily on the meta sites) and I'm not suggesting that change; users would merely have that same option extended to them for all tags.
The only drawback I see to this is the possible implications it would have on caching. That being said, if the tag recoloring was performed client-side after the page loaded that would likely reduce the server overhead considerably.