New pawn promotion variant

So I had an idea for a chess variant which had an additional promotion rule for pawns. Basically, if a pawn captures another piece, it will become the piece it captured. So if you capture a knight, your pawn becomes a knight and so on. Obviously if you capture another pawn you remain a pawn. If you manage to have a passed pawn promote normally then normal promotion rules still apply.  I think that would lead to interesting strategies where either you're hesitant to make a trade knowing that you're promoting their pawn, or else on the opposite end if someone has a passed pawn that they want to promote to a queen, if you defend the promotion square with a knight/bishop you can try to force them to promote it down instead. What do you think?

This principle is of promoting to what you capture is known as 'contageon'; what you propose is to make all pieces contageous, but only for pawns. (I.e. all other pieces are 'immune'.) I used that rule (borrowed from the historic large Japanese chess variant Maka Dai Dai Shogi) in my variant Werewolf Chess. There the only unorthodox piece (the Werewolf) is contageous, and only Kings are immune. The effect is to make it next to impossible to trade the Werewolves out of the game (and thus revert to normal Chess).

In general having pieces promote when they capture is a very strong deterrent for trading those pieces. This might paralyze the game, especially with FIDE Pawns, which tend to form impenetrable interlocked walls if they are pushed without trading. (In Maka Dai Dai Shogi pawns move and capture straight ahead, so the danger of a stand-off is smaller there.)

Is there a game of Werewolf Chess anywhere to view the moves?

I think it's interesting as a variant.

Compared to classical chess it probably makes the game a bit too 'liquid' - as in the key squares of the position will change too radically to reach the depth acquired by playing deeply intertwined moves in order to build up tension throughout the structure.

Also, piece sacrifices against a pawn can sometimes be good for the party that is left materially down. And this type of new power play changes that sort of tactics too much I feel.

By the way, I also have an idea as a small addition to pawn promotion: King promotion.

Abiding by the new rule, pawns are now not only able to promote to a bishop, knight, rook, or queen, but also to a king.

Firstly, if a nation can have multiple queens, multiple kings should be possible (vice-kings).

Secondly, this can help prevent draws because having two kings means that suddenly no more checks are possible. This prevents perpetual.

So to check with two kings, first one king has to be captured, checkmated, or stalemated, then the other king can be checked. The win only happens when all kings are conquered.

Thirdly, if all you need to win through promotion is a piece that can wall off an enemy king on three adjacent squares, a king can be a more elegant option than a queen and can also prevent draw by stalemate.

Fourthly, it adds some complexity to the endgame which should make it even more interesting.

I think it would be hard to keep track of which pieces are pawns and which pieces are not

It seems you misunderstood the original poster. Because this is exactly what he proposed, changing into the victim on any capture.

And it will dramatically alter the game. But it seems to make it more difficult to attack; it would be hard to trade away pieces protected by a Pawn.

Oh, I see. Yes, I misunderstood.

I see your point. That really would change a lot.