Yo man,
I think Plus! would be great if you added scripting to it so that we can play a text version of Quake. also, if you could support DirectX it would be nice. A must have would be to be able to send mpeg videos in real time to any contact, even when they are using a slow internet connection.
Of course, you should add all the features of every other Messenger add-on, I'm sure it wouldn't be that complicated. Maybe you could try to think again the Preferences windows too, they could be a little more attractive and easier to understand. Don't take me wrong, I just want to help.
Oh yeah! if you could add ICQ style logging and about 200 more emoticons that would be so grrrreat! thanks man! we're all waiting for improvements in Messenger Plus!