What do you mean by customising themselves via uploading classes? Actually loading an external class wouldn't be too much hastle, but what would you actually let people do with this?
Well, I didn't think it through, but something in the lines of how being able to subclass the class which defines how you are presented on the client. Something limited enough to be safe but powerful enough to be fun. Sort of like an applet so to speak if you get my drift.
I'm sorry to be so vague, but like I said, I didn't think it through.