I run a community site for authors and artists, and I would love to create custom URL fields for their profiles to lead off to their accounts on Blogger, LiveJournal, deviantArt, Twitter, etc.
What I would like to see is a new field type which allows me to create a URL template/expression that is then used to wrap the user's input in a link using that expression.
Say on the back end I create a field called "deviantArt", and build the template for it as
[code:1]http://{input}.deviantart.com/[/code:1]
When editing his profile, John sees the dA field, and enters his username for that site, "johndoe123". After saving, his profile page (or contact info tab, or custom tab) shows "deviantArt: johndoe123" with his name linked to
[code:1]
johndoe123.deviantart.com/
[/code:1]
Optionally, the administrator could also upload a thumbnail image to be displayed alongside the field name.
There's already a plug-in out there which does something similar to this (CB Social Networks), but it's clunky and not very customizable.