Fluid Chess

Fluid Chess was inspired by a few ideas about boards and pieces per square, playing some of Jeremy Good's games, and Fergus Duniho's Fusion Chess.

Setup

The setup is standard FIDE.

Pieces

The pieces are standard FIDE, except where otherwise noted.

Rules

This game differs from FIDE only in movement rules.

Pawn movement rules are unchanged.

No piece may jump. Knights lose their jump and become lame. However, they have 2 paths to any destination square: they may move one square orthogonally, then one square diagonally outwards, or they may move one square diagonally [thus capable of landing on all 8 adjacent squares on the first step of their move], then one square orthogonally outwards. [The usual term for this piece is Moo".]

A friendly piece may freely move into, out of, or through a square occupied by any other single friendly piece [not pawn]. There is no limit, other than practical, to how many times this can be done in one turn. For example, on turn one, either player could move their king's rook through the knight and bishop into the king's square.

Neither pieces nor pawns may ever move into and merge with, or move through, any pawn[s].

If two friendly pieces start a turn on the same square, they may act as a compound piece, both pieces moving together to a legal destination square of either piece. So a bishop and knight would be able to act as an archbishop/paladin. Two of the same piece gain no additional moves.

If two friendly pieces start a turn on the same square, one may move away as if it was alone on the square of origin for that move. This moving piece may end the move in another friendly piece's square.

A queen is already two friendly pieces on the same square, bishop and rook.

Castling is done a little differently. As no piece may jump, the king and rook slide into their positions. Either may move first, sliding to its position; then the other slides to its position. Both may slide through other pieces, and the second to move may land on a friendly piece. But the first piece to move must end its slide on an empty square, because the second piece to move must slide through the first piece's current square, and if there are already 2 pieces in that square, the move is illegal.

The changes in movement affect capture. All capture is still by replacement. Self-capture is not allowed.

A single pawn or piece, moving into an opponent's square, may only capture 1 piece in that square. If there are 2 pieces there, the moving player chooses which non-moving piece is captured.

A pair of pieces, moving into an opponent's square, may capture 2 pieces in that square.

Optional Rule 1: Promotion may only be to one piece. A queen is two pieces, by definition. Therefore, promotion may only be to a rook, bishop, or knight.

Optional Rule 2: Promotions may [only] be to doubled pieces, specifically the Queen, the BR combination; the Paladin, the NB combination; or the Chancellor, the RN combination.

Notes

This is a modest variant; it's an old wargame concept of stacking, brought into chess. I'd been thinking of squares that split into 2 levels when a piece occupied them, allowing occupancy by 2 pieces, but never got much past that. * Then I started a game of Fusion, and saw Mr. Duniho's game was an excellent vehicle for expanding on the idea. This mutator is the result. It can be applied to other games.
* Somewhere between these two sentences, I played some moves in a number of different games by Jeremy Good which somehow contributed to the process. And I'm not sure how.