I know you are working on the first one of these already. But figured I'd post the rest as well.
Ability to have "offline" be the chat default unless a user marks themselves as online. We are writers, and when we're writing we want to be left alone. Sometimes we don't notice chat is on. OR, if we switch pages to research something our writing partner wrote, chat reloads and shows us as online again until it resorts itself and goes back offline. So then we get people convinced we're ignoring them. It's not great to have a site of creative people if they are all getting upset over things like feeling ignored by potential writing partners. It should be offline no matter what until the user specifically marks themselves as online.
Smileys to match the site's smileys.
Ability to resize the "Who's Online" list height-wise as it really is obtrusive on our site and covers information we don't want covered.
Turn off the chat tab from showing on AdminCP footer.
Ability for user created groups to be deleted after, say, 10 days without any activity.
Ability for regular users to BLOCK chats from someone. We have users who have blocked people from contacting them on the site itself, and yet these people are contacting them on chat and they can't stop them from being able to send the chat messages. This is a cyberbullying issue. As a social networkin site, we NEED to be able to let people choose not to recieve messages from certain people.
Ability for the user to save the conversations. In most chats, you can hit "save conversation" and it'll download a txt file of the conversation. Again, for bullying, this is a much needed, necessary feature. Also a lot of us pre-write posts, or storylines, or discuss ideas for our characters, etc in chats. Being able to save these items is a must.