This sounds amazing.

I've actually thought about this in the shower on multiple occasions. Definitely a bit of kinks to work out, especially considering check/checkmate

This is what you think of in the shower? Lol :P.

For check and check mate, how about if a player gets in check one turn, they have the next turn to move away, but if they are checked again next turn it's checkmate. Pretty much, two checks in a row= checkmate.

Can you move every piece at once or can you only move a small subset of pieces? Keeping it to 3-5 moves might make it less daunting of game. Really cool idea though. I'd be interested in doing this as a PBEM.

All the moves, every piece, at least that's the way i imagined it.

As for check, i'm not sure, you're probably in a position where moving the king risks getting it killed without ever being in check - infact with simultaneous moves i don't think you need to have rules for check, though checkmate would still be a guarenteed win; i think even with support holds this is a very offensive game (because 1vs1 becomes a swap, except for pawns, and holding is a great way to get yourself killed...) Having a king sit behind a pawn wall and support holding them would help somewhat.

I don't know, i haven't thought about how much pressure/support can be brought on a single square or if stalemates can be formed...

Also, are there support moves? If there are support holds, there probably should be supported attack.

What happens if two pieces order a switch in position or are ordered to cross each other's position?
i.e. Let's consider rooks on A2 and A7. What happens when the orders are:
a) A2->A6, A7->A3
b) A2->A7, A7->A2
My guess is that they should bounce and both remain at A2 and A7 but, who knows? Playing the game would be a good way try see what works.

Seems like it could get pretty complicated when the beauty of both chess and diplomacy, imho, are their fairly simple rules. As a first go round, I'd love to try it out with just a few simultaneous moves and rules like you suggested. A stationary piece gets taken, two moving pieces bounce.

Yeah, can't have support holds without supported attacks.

Playtesting would be useful. I think only knights should be able to swap places - with hmm... i'm torn between bounces and mutual destruction.

Move order will be very important in some cases - if you castle (a king move), you will need to resolve that before the rook move is resolvable.

A castle would have to be a move for both pieces, I think

Two more questions
1) Can you only order legal moves or can you take a guess at an illegal one?
i.e. Opponent has a piece on A2, your order A7-A1 hoping it moves
2) Are pawns pretty useless now? It is pretty obvious when a pawn can take a piece, so you'll simply move out of the way. This could be fixed in two ways i) you can only move a few pieces at a time or ii) illegal moves are allowed so a pawn can be ordered to take a piece in anticipation of it moving there

I like allowing illegal moves - basically threatened squares should work in the same way as in normal chess.

Other illegal moves like moving into check should be allowed - because with the king being able to move back out of check on their next turn it is not completely suicidal - however, if you move to what would be check and they also move a piece there taking your king you should lose the game right away.

As for the first part of your question; i think taking 'illegal' moves and allowing them fail would be interesting. In your exmaple, Leaving that piece at A2 would be risking it being taken, whereas keeping it there would prevent the opponent moving past...

That said, if we allow illegal moves like this, then what happens if you move out of A2 with the piece which is there and into A2 with another piece? I'm going to assume no jumping (except the knight) So the move could either fail - because there is something blocking it - OR pass by A2 because while all the pieces are moving there is nothing there to stop it...

I'm not sure.

The way I would lay out the first game is like this:

1) You can order any 5 moves. Let's say castling counts as 1 move by neither the king nor rook can be ordered another move.

2) Two pieces ordered to the same spot or through each other bounce.

3) A piece ordered to a stationary piece captures it

4) Any move that fails or is bounced (i.e. remains illegal) is considered a stationary piece.

5) You win by capturing the opponent's king

Here's an example of how I think the rules would play out:

-White has a piece at B5.
-White orders his pawn A4-B5
-Black orders a piece to B5 and another piece to A4

The two pieces at B5 bounce (and white's piece is safe). However, black captures white's pawn at A4.

Wow, great idea!

How about pieces need to be dislodged Diplomacy style rather than destroyed so easily? Makes pawns more useful and interesting since it takes quite a bit of effort to.destroy them.

Great idea indeed, btw.

i think pawns given support to hold would be nice because it would give us the usual chess positioning of pawns supporting each other....

i like Yonni's suggested rules, with the one addition of supported moves.

So when a bounce would occur but you have a second piece able to move to that square then you can order that piece to support the move, and you still take the position.

And when a stationary piece would be taken you can support it to hold with any piece which can threaten that square (so a pawn moving diagonally but not forward, and any other piece which can move to that square)

With stationary strenght counting as 0, that means 1 support boucnes one attacking piece; whereas 4 supported attacks on one piece with 4 supports to hold will still result in the stationary piece being taken. (that should avoid stalemates, i think... depending on positioning.)

And I assume supports are cut as per usual?

Yes. ie especially if a piece is taken.

The concept of "check" and "checkmate" might not make too much sense or really be necessary in this type of game.

Depending on the exact rules, the simultaneity of moves could result in a king being captured inadvertently even when it wasn't under threat in the previous position. You could also have cases where one player has a 50-50 shot of capturing the opposing king, or where both kings get captured simultaneously or are simultaneously put under threat.

I think it would be better to just play until one or both kings have been captured. Simultaneous king capture would be a different way to draw (perhaps call it "mutualmate").