1. E4 is best by test. No special tactic or further transparency necessary.
21st Century En Passant?

Better to simply abandon classic chess and go to all chess960 all the time. Eventually computers will work out all viable lines possible even with the 960 possibilities then it will be time for another change which would be to randomize both sides as in 960 but do it separately for each side. By then global warming will have done its job and there will be no more chess related matters to be concerned about.

I rather like the idea - but not as a secret.
Secrecy would make the game too unpredictable and un-chess-like. Though it would be a bit fun to see how opening theory is altered, it would eliminate whole classes of attacks. You'd never dare to smash through a pawn wall with a rook, and you couldn't even use a knight or bishop without extra protection. (Figure the super-pawn is worth three or four ordinary pawns.)
Intead, each side gets a sticker they can place on a pawn. Maybe white chooses first? Or maybe the choice is randomized as in 960?
Like the NFL moving extra point attempts back, is it time for another rule change in chess like when en passant was added? Football is not meant to be played by robots like chess is not meant to be played by computers. What if before the game you get to select a secret poison pawn that gets to move in all directions like a King and it doesn't have to be revealed until making a backwards or sideways move? Will man or machine get a virus with this rule change?