Another theme idea: having unequal armies on the light and the dark squares.
A player might have pieces of type A, which can move only on light squares, and pieces of type B, which move on the dark squares.
The opponent may have the same types and numbers of pieces, so that the armies are equal over all. The opponent might also have his A-pieces on light and B-pieces on dark squares, so that the A-pieces fight each other, and also the B pieces, but an A piece can never meet a B-piece. It's also possible to make the setup vice versa, so that on any square color the A-pieces of one side fight the B-pieces of the other.
Another theme idea: having unequal armies on the light and the dark squares.
A player might have pieces of type A, which can move only on light squares, and pieces of type B, which move on the dark squares.
The opponent may have the same types and numbers of pieces, so that the armies are equal over all. The opponent might also have his A-pieces on light and B-pieces on dark squares, so that the A-pieces fight each other, and also the B pieces, but an A piece can never meet a B-piece. It's also possible to make the setup vice versa, so that on any square color the A-pieces of one side fight the B-pieces of the other.