I really like this variant!
Variant idea: Board Shrinks Every 20 Moves

This game starts on a 15x15 board where each side has 15 pawns, 4 rooks, 4 knights, 4 bishops, 2 queens, and a king (the pieces will start on the 2nd, 3rd, 13th, and 14th rows). Starting from move 20, the board shrinks such that the outermost edges of the board disappear. Any pieces in this area get removed from the game. If one player's king gets removed, the player loses. If this happens to both players, the game ends in a draw.
The 15x15 board will turn into a 13x13 board on move 20 and a 11x11 board on move 40. If the game didn't end already, the board will not shrink further after it turns into a 5x5 board.
All usual rules of chess apply in this game.
what about 10 moves? 20 is too much because no games end on move 200
Oh yea

This game starts on a 15x15 board where each side has 15 pawns, 4 rooks, 4 knights, 4 bishops, 2 queens, and a king (the pieces will start on the 2nd, 3rd, 13th, and 14th rows). Starting from move 20, the board shrinks such that the outermost edges of the board disappear. Any pieces in this area get removed from the game. If one player's king gets removed, the player loses. If this happens to both players, the game ends in a draw.
The 15x15 board will turn into a 13x13 board on move 20 and a 11x11 board on move 40. If the game didn't end already, the board will not shrink further after it turns into a 5x5 board.
All usual rules of chess apply in this game.
what about 10 moves? 20 is too much because no games end on move 200
Actually, since the board goes from 15x15 to 13x13 to 11x11, it will take 100 moves to get to a 5x5 board. By that time, the game probably would have ended, but there is still a chance it goes that far
But, I do think 15 moves might be better
What rank is a promotion rank?
In XXL chess, the promotion rank is 9th rank(standard) and Gas Gas Gas to 15th rank is too far for pawns.

Wow that's funny I invented this previously... but that's ok, whatever it takes to get this created is good with me. For almost a year now I have been suggesting a shrinking board game to 4pc but since they ignore everything I gave up (4pc and the idea). Yeah I would totally love to see this made a reality! Hope they actually consider this. It may need tweaking but overall it should be good and fun.
This game starts on a 15x15 board where each side has 15 pawns, 4 rooks, 4 knights, 4 bishops, 2 queens, and a king (the pieces will start on the 2nd, 3rd, 13th, and 14th rows). Starting from move 20, the board shrinks such that the outermost edges of the board disappear. Any pieces in this area get removed from the game. If one player's king gets removed, the player loses. If this happens to both players, the game ends in a draw.
The 15x15 board will turn into a 13x13 board on move 20 and a 11x11 board on move 40. If the game didn't end already, the board will not shrink further after it turns into a 5x5 board.
All usual rules of chess apply in this game.