I agree everyone would see the desired changes from their own point of view and usage of mIRC.
Let me add few from my point of view:
* as you write spell checker similar to Office XP (I cannot spell),
* tree view list of open channels and queries instead of buttons (I know other clients already have it)
* serialized ident (I know I'm the only one on the planet who wants it)
* some method of protecting scripts against "script viruses"
* channel list to include clickable, resizable headers for sorting, showing modes as separate column)
* support for SSL chat (as provided by ConferenceRoom ircd and some others)
say what you want but I think these would be major changes.