SMF could use an Admin function to add custom profile fields, in a way that survives upgrades.
Possibly, the fields already exist, even if you never use them. In Admin, you choose what to name the custom fields you use, whether they are required on registration page. Possibly allow Admin to re-order them.
That's something I'm looking for as well.
I'm setting up a motorcycle forum and would like to have a place where users can list their motorcycle in their profile and then have that be shown with each post.
I'm still looking through existing mods but haven't seen anything yet.
http://www.simplemachines.org/community/index.php?topic=494.0