The only thing I'd like to see is a switch or maybe even new keyword to run a while loop that will execute on a different thread and will be destroyed when the loop is finished, preferably where everything is managed for us. Something similar for the $findfile/$finddir identifiers would be useful too.
I think full control over threading is overkill.
I agree that putting while loops into a separate thread would help to avoid freezing mIRC's main window while running scripts. Of course, if you don't see mIRC freeze, it's hard to know if there is a broken while loop. I'd assume there would need to be some method of seeing that. I don't know a ton about multithreading, so just noting what came to mind.
