(Of course I'm sure this has been suggested already :P)
I think that there should be more dialog addons such as treeviews, dropdown lists with icons, etc. In the same kind of fashion that some of the popular dll addons such as MDX and DCX allow you to do. On top of that, the dialog should be a bit more customizeable, for instance being able to change the colors of various dialog id's.
It just makes sense to me, since lots of scripters wind up using these dlls to make their dialogs more 'user-friendly', that they would be built in features in mIRC.