What about Hulk??? When it gets angry it just smashes its way through the board and kicks the king on the face.
(P.S. I am not responsible for your board and pieces.)
instead of an extra piece perhaps each player could have an extra cardboard square or two they could add to the side of the board if they need it to get out of a sticky situation. It can be called anything you like if it gets you out of trouble.
Portals. They replace rooks, but only move one space at a time, and can't attack. Pieces can go through one portal and continue their move out the other, in the same way. Knights make a knight move from the second portal. If one portal is destroyed, a turn can be spent to replace it. It reappears next to the portal in play. Both portals need to be removed, to remove the portals from the game.
Have you ever played "bughouse" or "siamese" chess? The concept is that in lieu of moving a natural piece that you can deploy one of your opponents captured pieces instead (after color change to your piece) dropping on any unoccupied square.
That is a very dynamic game to say the least.
Casanova: He can only take Queens and pawns and can only be taken by queens and pawns. If Casanova is taken by a Queen, you get a pawn wherever you would like it as long as the other person is not put in check because of it. If Casanova is taken by a pawn, you get a Queen that you can place anywheres as long as the other person is not put in check.
How about an Asian piece that sits a few tables down from you with these gorgeous legs that just...well...we will call that piece a "Distractor." She moves like...(hard to describe in words) and she has this tactic called "Eye Diversion." Very powerful piece. Not sure you can promote a Pawn to it, and not sure if my opponent placed that piece over there, or a previous opponent that I had played before that wanted revenge on his loss. Not sure how to capture her.. It's the most powerful piece on the board, though, because...it's not on the board. It makes you keep looking, and it's sole objective is to get your mind off the game so she can use these amazing tactics on you that will cause you to lose a rook and possibly...your own Queen. I want to get one for my opponent! "I lost me Queen? Did I? Well, how about nice a Distractor!! TAKE THAT!" lol.