I've been thinking/wanting to build this too for my RPG forums. I've built a couple extensions for 3.1 so far for our forums and I have a few other things to code with higher priorities, but this was on my future to do list.
I wouldn't use accounts, but I was thinking more of having characters/aliases, etc. Because then it would be easier to create NPCs and switch/move/copy character data between users if a user quits or another player takes over that character. And you wouldn't have to worry about passwords/maintaining multiple user records, account validation, hacks, etc.
Then when the user posts, it would have a dropdown picklist to select which character/alias they were posting as.
The character\alias would have a name, avatar, and background\textarea associated with it accessible/editable via UCP. So users can click the character name\alias and see a summary\avatar\background of the character. Char sheets would be too complicated, plus we host a dozen different systems, so just a textbox big enough for a small bio/description would be great.
I'm curious to what other extensions that the other play-by-post forums are using. We're using @Mentions and Recent Topics just as normal board stuff. Then we've built our own Dice Roller and Games List extensions at http://www.dicefellowship.org