If when you click someone's name who is offline, besides 'Remove' there could be an option to tell you when they were last online. This would be particularly useful in groups that only allow active members, or groups clearing out old members.
Another idea I was thinking about but this idea I was unsure about, because I'm not sure if it's worthy of mentioning. But if you are in a roleplay, the scroll bar on the side could have a small blue tick wherever whispers occur. It's easy to loose whispers from friends if you are roleplaying, and this could help you keep track of them. Again, I was unsure of this, and if you viewed the whisper the tick would go away.
I'm not too sure about these but it's worth a shot asking about, right?