don't forget an "on drclick"
Also, to add some things to the idea.. What about drop menus? That'd kick ass.

(the type I mean is windows style drop boxes that open on right click, where the mouse is)
How about a few javascript style addons? on mousedown (probably renamed to "on mdown" for mIRC). On m(ouse)up, on m(ouse)click.. )
More additions:
Picture placement would be nice.
If a picture window or regular window could be placed in a dialog.. oh man, that'd be beautiful.

Custom buttons, where a "regular" pic for the button can be specified for when it is not clicked or highlighted, a picture for when while it's being pressed, one for when it pops back up (if the button wasn't triggered, if this value isn't specified, then it will use the "regular" pic), and one for after it's been clicked. (again, if this isn't set, it will revert back to the "regular" pic) Plus whatever I haven't thought of here.. (What if someone clicks the button, thinks about it for a second, then drags the mouse away from the button, all the while holding the mouse button down, as to not click it?).
The abillity to store a dialog's framework in a file and load a dialog from a file. Then it would be possible to have dynamically constructed menus that can be changed via scripts or replaced even.. all without updating any scripts. Dialogs wouldn't have to be so.. rigid, then.
A suggestion that would be nice:
The ability to open a dialog in "positioning mode". When a dialog is opened in "positioning mode", you will then be able to drag objects around, resize them, or perhaps even set other options. When the dialog is closed, it asks if you want to save the changes or not. (the dialog will not function as it normally would while in positioning mode. No events will be triggered, unless otherwise specified.) Perhaps certain events could be set to be allowed. To do that, it'd have to be listed in the dialog table (ex
dialog [-l] name {
title "text"
icon filename, index
size x y w h
option type (dbu, pixels, notheme)
posmode event,N
text "text", id, x y w h, style (right, center, nowrap)
... etc, etc..
N would be an ID number
event is.. well.. guess (ex. sclick) lol
Right now the only alternative is some kooky script or change 1 thing on the dialog, open it, change another thing.. close it.. open it.. bah.. what a pain in the buttocks.
Things like this would take mIRC up a few notches, I believe.
If you've managed to read this whole message, I applaud you, and apologize for the length. lol