I have never seen this on any journaling service. I know squeaky is not suffering from lack of suggestions, but here goes:

Some sort of option or script whereby, whenever a user posts or comments, the userpic is a random one. That is, the script chooses it randomly from all that user's icons. (If the notion of icon "groups" ever gets implemented, it might choose from only one or a few chosen groups).

I have no idea how much work such a feature would involve, and it's hardly an urgent need, but I thought I would toss that out.