I would love to see fields built into the bio for credit for one's avatar and header. Even if bio length isn't a limitation, it would be cleaner, consistent across profiles, and serve to encourage people to credit artists/photographers/meme-editors/etc for their work and discourage image theft.
Ideally to me, it would just be two fields, represented by an icon, that automatically detect links, ie:
This seems like a pretty neat and sensible idea. I like it.