(I'm aware this has been mostly done before, but I don't know of anything that offers this exact combination of features.)
An IRC (and possibly other messaging systems) client separated into a daemon and a set of frontends, supporting multiple servers, and (more importantly) multiple configurable views. Currently I run lots of copies of irssi, each connected to one or two networks, so I'm stuck with the irssi text interface; what I'd like to be able to do is use a graphical interface with a load of predefined views ("UKC", "programming" etc.), and have a text client that I could run from screen that would display the same information. The daemon should also support scripting.
Chris Ball pointed out that ERC inside XEmacs pretty much meets all these requirements, since you can attach to an existing session using gnuclient.