I really, really like the "secret passages to other squares" idea proposed by a member in this thread earlier. Like a portal or warp zone.
Never mind. The more I think about practically using it, it seems like it wouldn't be any good. You'd have a perfect position until some weaker numbskull uses the secret passage cheapo to gain an advantage.
Cue Nimzowitsch, "Why must I...."
I have played with a portal variant. The rules I use are:
Rooks cannot capture pieces or get captured. Rooks lose their initial movement. Rooks can move to any square another piece of the same color can move to. Rooks is not allowed to move when you are in check. A piece can move to a rook and continue in the same direction from the other rook of the same color.
Note: Castling counts as both a rook and king's move, so castling is no longer legal. Pieces can move through rooks regardless of color. Knights can also move to a rook and continue in the same direction from the other rook (meaning that they will move in an L-shape twice during the same turn).
what I just said is better
There are invisible portal squares that transport your piece(if you put it on the square) to places from the 3rd rank to the 6th rank.