i think being able to choose the text on the buttons would be good, like in $sdir you can choose the text on the buttons.
Not possible. mIRC simple calls a function predefined by windows and sends it the necisery information. Im not sure in C\C++, but im sure its very similar to C#. You call a function MessageBox.Show("Text", "Title", MessageBoxButton.[OK|OKCancel|Cancel|YesNo|Etc], MessageBoxIcon.[Warning|Exclaimation|Etc]);
Basically, windows controls the dialog, and not mirc. The size, orientation, etc is done by windows based on what is passed. You cant change the text unless Khaled himself wrote new code (new dialog) so you can have new button text.
I think Khaled wrote his own code. In WinAPI there is no flag for MessageBox() that says "display a textbox in the message box" yet $input does have the ability to prompt the user for input. If thats the case, then yes Khaled could allow the text to be changed.
There could also be an individual exception for just the text box.