A disappearing corner square seems to be a useful concept. In my Doublebarrel Chess I introduce a similar idea. There are no extra squares, but the extra pieces are placed beside the 8x8 board. When a rook moves away, the extraneous piece is automatically moved to the corner square. Should a rook be captured when it still hasn't left the corner square, then the extraneous piece is removed, too. The Doublebarrel is so powerful in the traditional setup that it was necessary to put it beside the board so that it doesn't run amok in the opening.
Another game which uses diagonal corner squares on a 9x8(!) board is Bolyar Chess (a traditional Bulgarian variant). Here the player gets a special bonus if he promotes a boat on the extraneous corner squares. The boat is then promoted to General (=queen).
An important aspect of the gustavian board is that the extra corner squares provide shelter for the king. This means that it's less dangerous to initiate a pawn storm on the king side. As it's easier to get the king out of the way, this improves mobility of the heavy pieces that can be placed on the knight- and rook files to bolster the flank attack. This strategical aspect, which benefits fighting chess, is a bonus of the gustavian board. Leaping pieces, especially those with long-leaping camel moves, benefit greatly from the extra corner squares, while the sliding pieces have no use for them, except the queen, which is somewhat benefitted. The reason for this is that the leaping pieces can use the extra corner squares when maneuvering./Mats
Another game which uses diagonal corner squares on a 9x8(!) board is Bolyar Chess (a traditional Bulgarian variant). Here the player gets a special bonus if he promotes a boat on the extraneous corner squares. The boat is then promoted to General (=queen).
An important aspect of the gustavian board is that the extra corner squares provide shelter for the king. This means that it's less dangerous to initiate a pawn storm on the king side. As it's easier to get the king out of the way, this improves mobility of the heavy pieces that can be placed on the knight- and rook files to bolster the flank attack. This strategical aspect, which benefits fighting chess, is a bonus of the gustavian board. Leaping pieces, especially those with long-leaping camel moves, benefit greatly from the extra corner squares, while the sliding pieces have no use for them, except the queen, which is somewhat benefitted. The reason for this is that the leaping pieces can use the extra corner squares when maneuvering./Mats