In Drawback Chess https://www.drawbackchess.com you win by capturing the enemy king by capturing, and moving through check is allowed. Castling through check, and out of check is also allowed but if you try to castle out of check or through check then your opponent can take your king en passant by moving to a square your king passed through or the square your king moved from. For instance in this game
my opponent tried to castle out of checkmate by castling queenside, so I placed my queen on the starting square of the enemy king in order to capture it.
I think similarly on this site there should be a variant of capture the king, in which if your opponent tries to castle out of check or through check you can take their king en passant by landing on the square their king left from or a square their king passed through even if you aren’t attacking the square their king landed on. As a compliment to that I think there should be a variant for games that aren’t capture the king, in which it’s possible to castle through check or out of check so long as the square you land on is safe. For instance in this second variant, in this position
White could get out of check by castling like this
I also think there should be a variant, in which it’s possible to castle when there’s pieces between the king and rook, so long as those pieces are enemy pieces, with those pieces being captured. In this third variant if there was a position like this
Black could castle queenside like this
Which would capture the two white knights, and so it could be denoted something like O-O-Oxc8xd8
In Drawback Chess https://www.drawbackchess.com you win by capturing the enemy king by capturing, and moving through check is allowed. Castling through check, and out of check is also allowed but if you try to castle out of check or through check then your opponent can take your king en passant by moving to a square your king passed through or the square your king moved from. For instance in this game
https://www.drawbackchess.com/game/4d1b07a82a8e8097bb981fc242ebca36/white
my opponent tried to castle out of checkmate by castling queenside, so I placed my queen on the starting square of the enemy king in order to capture it.
I think similarly on this site there should be a variant of capture the king, in which if your opponent tries to castle out of check or through check you can take their king en passant by landing on the square their king left from or a square their king passed through even if you aren’t attacking the square their king landed on. As a compliment to that I think there should be a variant for games that aren’t capture the king, in which it’s possible to castle through check or out of check so long as the square you land on is safe. For instance in this second variant, in this position
White could get out of check by castling like this
I also think there should be a variant, in which it’s possible to castle when there’s pieces between the king and rook, so long as those pieces are enemy pieces, with those pieces being captured. In this third variant if there was a position like this
Black could castle queenside like this
Which would capture the two white knights, and so it could be denoted something like O-O-Oxc8xd8