Add a field to a chat where we can put the subject of the chat. I think You are also faced with the problem to find the right chat in the history. If there is a subject it is easy to find or remember. If it has ...
I suggest to integrate the file based chat history into the Notes client. The Chat history manu should present chat history content even if the user is using file based history.
It would be great to see if the other party of a chat was seen you message. Sometimes you send a message and nothoing happens. If "seen" status will be introduced then you will see if something happening on the other side.
eg: Facebook does it...