This suggestion is being made every year or so by you, it's... wonderful.
It would be breaking backward compatibilities and practically speaking break a lot of scripts, I have no idea why khaled edited your post, as though this was not at the top of impossible changes to make.
If it has to be stated then ok: I'm absolutely against this idea which would just break compatibilities for no good reasons: there is not a single argument as to why this would be a good idea to have in the scripting language in the first place.
And I believe there's no argument because it would be for convenience only, this would not serve any real purpose, it would not help with functionalities.
Please focus on what can be improved/changed in mIRC, %variables parsing is not one of them.

Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net