I don't know if this is possible or not, but I'd like to see an add-on where users can choose the user group (mark-up) of their choice based on the user groups they already belong to.
For example, say one member is associated with the following user groups:
- Registered (light blue)
- Moderator (green)
- Blue Subscriber (dark blue)
- Red Subscriber (red)
Here's an example of how it looks on vBulletin:
I've also requested this to be a core feature in XenForo, but I guess people don't rely on this functionality as much as I do for donations, etc.