Add a bunch of extra chess pieces (perhaps fairy chess pieces) to each side and set them up on a wide rectangular board of 8 ranks by N files. Keeping it 8 ranks continues the feeling of battle lines engaging each other. However, because the battlefield is very wide, it would normally take a long time for action on one side to affect the other side, destroying the unity of place. Pieces on opposite sides of the board don't coordinate well.
Fix this by giving every piece except pawns the ability to move arbitrary distances sideways like a rook (but not forward and back). Perhaps colorbound pieces can only stop on the correct color, or they can jump sideways like a dabbabarider.