But now I've got an idea for a couple of variations using the jokers. One version uses them as wildcards. A joker may stand in for any other card when building a column, but it must do so consistently; for example, if a joker is played onto a black seven, it represents a red six, and only a black five may then be played onto it. However, a joker may change what card it represents when moved to another column. Jokers may not be removed from the game or placed on foundations.
A harder variation uses the jokers as blocks. They may be placed on empty columns and in free cells, but they may not be removed from the game, they may not be placed on any other card, and they may not be built on. They are simply in the way, and they run the risk of clogging up otherwise useful cells or empty columns, so it's in your best interests not to uncover them too soon.