My current favorite of the variants I've designed so far is Cold Fusion Chess. It's sort of a hybrid of Grand Chess, Fusion Chess, Pocket Knight Chess (and an informal variant idea involving demotions that appeared in the discussion thread on Pocket Knight), and a little bit of Shogi.
As in Grand Chess. Additionally, each player has two places to put captured pieces: the Safe Pocket and the Demotion Pocket.
The pieces at the start are the same as in Grand Chess. There are additional pieces, though, which can appear as the result of fusions. The linked article on Fusion Chess lists most of them, but there are a few that are not found in either (although many can be found in other chess variants):
- Berolina Pawn — sort of the inverse of a pawn, it can move passively (that is, without capturing) one space diagonally forward, and capture one space straight forward. It is considered a kind of pawn.
- Sergeant — a combination of a regular pawn and a berolina pawn, it can move and capture one space in any of the three forward directions. It is capable of capturing en passant. It is also considered a kind of pawn.
- Knightrider — can move any number of knight's moves in the same direction until it encounters an obstacle.
- Archbishop — moves like a bishop, but may reflect once 90̊° off of the edge of the board.
- Chariot — moves like a rook, but may turn once 90° and continue as a rook when it encounters an obstacle (an edge of the board or a friendly piece). As far as I know, this piece is unique to Cold Fusion Chess.
- Cavalry, Berolina Cavalry, Cavalry Sergeant — may move as a knight or as a pawn, berolina pawn, or sergeant, respectively. Chess problemists call the Cavalry a Dragon, but Fusion Chess already calls the king + rook combination a Dragon King, and I wanted to avoid confusion.
- Templar, Berolina Templar, Temple Sergeant — may move as a bishop or as a pawn, berolina pawn, or sergeant, respectively.
- Guard, Berolina Guard, Guard Sergeant — may move as a rook or as a pawn, berolina pawn, or sergeant, respectively.
The same as standard chess, with the following changes:
- Instead of making a regular move, a player may pocket one of his own men by moving it to his demotion pocket—basically, capturing his own piece. The king may not be pocketed. A player can only do this if he could make a regular move (that is, he's not otherwise stalemated).
- Instead of making a regular move, a player may drop a pocketed or captured piece onto an empty square. Captured pieces dropped in this way switch their allegiance to the player who dropped them (as in Shogi). If the piece was dropped from the safe pocket, it remains the same kind of piece. If it was dropped from the demotion pocket, it becomes a different piece of equal or reduced value (see Demotions). Obviously, a player cannot do this if his pockets are empty. A player may not drop when his king is in check, even if it would break check. A pawn cannot be dropped on the 9th or 10th rank.
- Pieces captured by regular (non-pawn, non-king) men are put in the capturing player's demotion pocket. Pawns captured in this way are removed from the game. Pieces captured by pawns (or by using the pawn component of a pawn-hybrid piece) or by a king go in the capturing player's safe pocket.
- Pawns have the option of promoting on the 9th rank, and must promote on the 10th (this is the same as Grand Chess). Pawns may only promote to friendly non-pawn (note that berolina pawns and sergeants are considered pawns) pieces that the opponent has captured—when this happens, the captured piece is removed from the opponent's pocket and replaces the promoted pawn (without demoting), which is removed from play. A pawn cannot promote if the opponent is not holding any friendly pieces in his pockets, and therefore may not move to the 10th rank at all in such a situation, but may still threaten the opposing king with check from the 9th.
- A piece may move onto the same square as a friendly piece and fuse with it. For a pawn to fuse with another piece, the pawn must be the one to move, using its capturing movement. See Fusions.
- A compound piece (one formed from two or more other pieces) can split, moving a component piece away from the compound and leaving the other component(s) behind. The moving component moves using its normal movement (e.g. a bishop splitting off from a queen can only move away diagonally, not orthagonally) and cannot capture or fuse on the same turn. A pawn splitting off from a compound onto the 9th or 10th rank promotes as usual (optional on the 9th, mandatory on the 10th); however, a pawn left behind on the 9th by another piece splitting off from a compound does not have the option of promoting, and a pawn cannot be left behind on the 10th rank by a split. Note that the queen, cardinal, and marshal in the starting lineup are considered compounds and can split.
Fusions work similarly to the ones in Fusion Chess, but are a little less restricted. For one thing, pawns can fuse. However, there are still restrictions:
- Each piece has a "weight": all non-compound non-pawn men including the king have weights of 1, basic pawns have weights of 1/3, and compounds have weights equal to the sum of the weights of their components. No compound may be formed with a weight exceeding 2.
- No more pawns may be added to a sergeant.
- The king may not fuse with pawns and vice versa.
- Pawn + pawn → Berolina pawn
- Pawn + berolina pawn → Sergeant
- Rook + pawn, berolina pawn, or sergeant → Guard, berolina guard, or guard sergeant respectively
- Rook + rook → Chariot
- Bishop + pawn, berolina pawn, or sergeant → Templar, berolina templar, or templar sergeant respectively
- Bishop + bishop → Archbishop
- Knight + pawn, berolina pawn, or sergeant → Cavalry, berolina cavalry, or cavalry sergeant respectively
- Knight + knight → Knightrider
- Guard, Templar, or Cavalry + pawn → Berolina guard, berolina templar, or berolina cavalry respectively
- Guard, Templar, or Cavalry + berolina pawn → Guard sergeant, templar sergeant, or cavalry sergeant respectively
- Berolina guard, berolina templar, or berolina cavalry + pawn → Guard sergeant, templar sergeant, or cavalry sergeant respectively
Compounds with pawn components lose one pawn component (as if it had split out) when demoting. All other demotions follow this sequence: Queen → Marshal → Cardinal → Chariot → Archbishop → Knightrider → Rook → Bishop → Knight → Sergeant → Berolina pawn → Pawn. Pawns do not demote.
I think that's everything. Yeah, it's quite a bit more complex than regular ol' chess.
EDIT: clarification of splitting pawns and promotion, minor format fix, "turning rook" now "chariot"