Jekyll & Hyde provides a great twist with multiple suits of 5 cards each with the suits designated J for Jekyll, H for Hyde, or J/H for suits that count as both Jekyll and Hyde.

The unique mechanism in this version of Mystery Rummy is that there is a two-sided card - the 'identity card' - on the table next to the draw deck, which has Jekyll on one side, and Hyde on the other. You can change the identity by playing a "potion" card that lets you flip the identity card and draw 2 cards from the deck. This is important, because you can only play melds whose suit matches the identity that is currently showing and it also makes for a tense endgame as scoring is also affected.