Let's invent some very weird pieces
The Hypnotist: Moves like a guard but doesn't have to move to the square it captures. If it does this, the enemy piece is now friendly.
Maybe a piece that permanently freezes a piece. So it moves slow, but it "takes" without moving - maybe the piece turns grey. So nobody controls this piece- it just sits there. It's possible it can't be taken, too, so this just makes a "hole" in the board- a place where nothing can go or go through.
Bear. The bear is slow and steady moves like a king, when the bear see a threat though (in this case an enemy piece) it can move like a queen to capture it (Note you can use this "power" to capture you can NOT stop halfway in) this makes the bear a strong and versatile piece especially on an open board. I also think this piece could actually be used in a game.