user31596: The en passant rule for pawns can be thought of as an 'attack of opportunity', such as in D&D - the attacking pawn would have been able to take the moving pawn as it passed through the first space.
When thought of this way, an obvious idea for a variant would be to generalise this by allowing the same behaviour in all cases, i.e. if during their journey a piece passes next to another piece that could have taken it, the next player has the option to invoke 'en passant' and take the piece where it would have been. Knights would be immune, as they 'jump' and thus do not pass through intermediate squares.
My question is, does this variant already exist, and/or is there a reason why it wouldn't work?

user30545: There is a variant called en passant chess.
Description from the source.
En Passant Chess
In standard chess, a pawn moving more than one step (i.e. two steps) can be captured by another pawn en passant on the move immediately following the multistep move.
Let us extend this rule to all pieces:

We use knightriders instead of knights. The king is a bit special (see below), and we use the other normal chess pieces.
A piece making a multistep move can be captured by an opponent's piece on its way, as long as this capture is done on the move immediately following the multistep move.
A piece making a multistep capture can be captured the same way. The piece it captured is not returned to the board (i.e. the capture is not undone).
As a king only makes a multistep move in castling, and it is already illegal to move it through a checked square, it cannot be captured en passant. It may however capture another piece en passant.
A king may make a knight (not knightrider) move, in addition to its normal king move, to capture another piece en passant. It may not use this move otherwise, e.g. to capture another piece directly.
The knightrider only steps on the squares a knight-move away from one another. For example, a knightrider moving from g1 to d7 can only be captured at f3 and e5. We can say that the move unit of a knightrider is a knight move.
All other pieces step on all squares between the starting and ending squares, so their move unit is a king move. Of course, the move unit of a rook is a wazir move, and that of a bishop is a ferz move only.
The only multistep move a pawn may make is the two-steps initial move, but beware that in this game it can be captured en passant by any piece!
Other rules are the same as in normal Chess.

Written by Andy Kurnia. HTML conversion by David Howe.

31596: <p>The en passant rule for pawns can be thought of as an 'attack of opportunity', such as in D&amp;D - the attacking pawn would have been able to take the moving pawn as it passed through the first space.</p>
<p>When thought of this way, an obvious idea for a variant would be to generalise this by allowing the same behaviour in all cases, i.e. if during their journey a piece passes next to another piece that could have taken it, the next player has the option to invoke 'en passant' and take the piece where it would have been. Knights would be immune, as they 'jump' and thus do not pass through intermediate squares.</p>
<p>My question is, does this variant already exist, and/or is there a reason why it wouldn't work?</p>
 30545: <p>There is a variant called <a href="https://www.chessvariants.com/difftaking.dir/enpassant.html" rel="nofollow noreferrer">en passant chess</a>.</p>
<p>Description from the source.</p>
<h4>En Passant Chess</h4>
<p>In standard chess, a pawn moving more than one step (i.e. two steps) can be captured by another pawn en passant on the move immediately following the multistep move.</p>
<p>Let us extend this rule to all pieces:</p>
<ul>
<li>We use knightriders instead of knights. The king is a bit special (see below), and we use the other normal chess pieces.</li>
<li>A piece making a multistep move can be captured by an opponent's piece on its way, as long as this capture is done on the move immediately following the multistep move.</li>
<li>A piece making a multistep capture can be captured the same way. The piece it captured is not returned to the board (i.e. the capture is not undone).</li>
<li>As a king only makes a multistep move in castling, and it is already illegal to move it through a checked square, it cannot be captured en passant. It may however capture another piece en passant.</li>
<li>A king may make a knight (not knightrider) move, in addition to its normal king move, to capture another piece en passant. It may not use this move otherwise, e.g. to capture another piece directly.</li>
<li>The knightrider only steps on the squares a knight-move away from one another. For example, a knightrider moving from g1 to d7 can only be captured at f3 and e5. We can say that the move unit of a knightrider is a knight move.</li>
<li>All other pieces step on all squares between the starting and ending squares, so their move unit is a king move. Of course, the move unit of a rook is a wazir move, and that of a bishop is a ferz move only.</li>
<li>The only multistep move a pawn may make is the two-steps initial move, but beware that in this game it can be captured en passant by any piece!</li>
<li>Other rules are the same as in normal Chess.</li>
</ul>
<p><em>Written by Andy Kurnia. HTML conversion by David Howe.</em></p>
