Abilility to mark anyone in chat with a badge.
This could be a customizeable badge that only shows up locally (other users cant see it). There are a few ways this can be useful such as:
1. Being able to mark friends so you can see if they are chatting in the same room as you.
2. Tagging yourself so your own messages can be seen easier in screenshots.
3. Tagging the person you are chatting with in a busy channel so you can easily see their messages before they get buried under lines of spam. (Ablility to make it temporary maybe)
If you need to you can have it saved as a browser cookie so it is stored on the users machine so you could save server storahe space. Also if the badge is not customizeable there will eventually be a userscript that will let people change it.
You could get BTTV and add their name as a Highlighted word... its not quite the same but it will work for you