Hi...I thought of this idea today with many similarities but some differences to everything that's been posted on here. Still trying to hash out a few things, however...
https://www.reddit.com/r/chess/comments/rdf9dy/i_introduce_the_new_game_unknown_chess/
What if one player wants to capture a piece and the other player wants to move the piece that is being captured? Which move will go first?
This is also a good example.
how is that a good example? There's no confusion here. Both pieces move simultaneously, so in that case, a capture doesn't take place. If you want you can even animate this out by hand. One hand on the capturing piece, the other on the in-danger-of-capture piece, then move both simultaneously, and as long as the in-danger-of-capture piece makes a legal move, it won't be captured. It is captured, however, if it doesn't make a legal move or it doesn't move.
I thought of this simultaneous/synchronous chess in the shower just now as well (hence looking it up now). I think it's a great idea. It would be a fun variant. It's like a discrete sampling of a real-life battle. Each player gets, say, 10 seconds per move (or make no move at all). Each players move (or lack of) is revealed at the end of the 10 seconds. Then 10 seconds to make next move etc. If you make an illegal move in that 10 seconds, then that move is cancelled and you essentially made no move. A player loses when their king is captured. A game can also end in stalemate when a player has no legal moves.
@7Deadknight I wrote up the game you suggested call Ven Chess its on Chess variant https://www.chessvariants.com/diffmove.dir/ven_chess/ven.html
The element where crossing of pieces is blocked adds to the game play.