As great as this one sounds, it's EVEN BETTER!
Your spectator freely selects ANY CARD from the deck and signs it, then puts it back somewhere in the deck. The top card is removed from the deck, shown to be a different card than the one selected by the spectator, and that card is placed in a special see-through card holder.
Now the AMAZING part- the card in the holder VISIBLY CHANGES into the spectator's SIGNED CARD at your command! Yes- WOW! This is an INCREDIBLE and UNFORGETTABLE effect!