Okay ... returning the piece to the opposition when used ... there are a couple of rules which need to be defined and they might read:

1. When an opponent's piece is captured (excluding pawns) it is kept by the captor for possible later use in accordance with rule 2. [Explanation: the King can already move as paawn so keeping them is of no value.]

2. A King may escape from check by taking on the movement (imposting) of a captured opponent's piece so long as the move lands the King on a square which is not in check and the King does not pass through any squares on the way which are under threat. On the next move, as their move, the opponent may place the piece imposted in the previous move anywhere on the board except a sqaure which would place the King in check. If the piece is not placed, but another piece is moved instead, the piece remains with the player for later imposting again. [Explanation: There is a heavy penalty for escaping from check by imposting a piece, the opponent gets the opportunity to return it to their ranks, however, this opportunity must be exercised immediately otherwise it is lost. A situation may arise where it is not prudent to replace the piece on the board as this may give the player a "free" move to exercise their own checking manouvers while not being hounded by continuous checking.]

Probably needs a little more tidying up but I think this gives a fair balance and introduces some interesting strategic possibilites.

Whisperz
Imposter Chess© Co-Exec