New rule = You may not move a non-pawn onto your opponent’s side of the board (or offer a draw) until all your non-pawns have moved at least once. 
There goes 1. f3 e5 2. g4 Qh4#,
And 1.e4 e5 2.Nf3 Nc6 3.Bb5.
What about moves to other squares on the first rank, such as Ra1-b1? What if there is a subsequent Rb1-a1? Does this rook count as having moved? What about castling?
Bill Smythe
- e4 e5 2. d4 d5 3. exd5 exd4. Is 4. Qe2 check or not?
I’d have to say no.
Furthermore, if black’s queen had been on d6 instead of d8, he could have responded 4…Qe5, and neither king would be in check, neither queen would be pinned on the file, and neither player could play QxQ.
Bill Smythe
In the following position, with white to move, does Bg2 give check, on the grounds that all back-rank pieces have now moved, whereas black was not in check before, because white’s bishop had not yet moved?
Bill Smythe
Check-'em and wreck 'em.