There are currently no "approved" substitutes for the Rook.
The Rook is a long-range piece that goes to squares of either color, starts off hiding in the corner, and becomes more valuable as the board becomes more empty.
It is possible to find pieces of roughly the same value as the Rook, but they will not be like the Rook. This difference will upset the dynamics of the whole game, and possibly change the way all the pieces relate to one another.
Bearing that in mind, two experimental pieces are offered:
[November 1996] Note that the LF probably wins material by tactical means right out of the opening position:
1. (LF)b1-c4 or 1. (LF)f1-e4 threaten mate at d5 or e5, and also threaten (LF)-b5 attacking a8 (and perhaps a8 is undefended).