A few issues:
1. This would take up screen space and make posts unnecessarily smaller. Not a problem on your PC screen maybe, but it's a concern for our mobile users.
2. The forum code is difficult to mod, and the online list is set up in a way that makes it even more difficult. There are reasons for it, mostly to keep them running fast, but it makes changing these things difficult. At the point where that list is being build, the name, colour and profile URL are (I believe) the only things we have. We can't change that without slowing down the forums.
3. We only have big avatars for pictures.
4. I don't see why we need it.
That last one is especially important in warranting me spending time on these things.