Either Black Box Pawns or Clue Chess could also be extended to replace the queen of the starting array with a "potential piece". This would add some variety to openings as well as endgames, which are the most thoroughly analyzed parts of chess and therefore the most due for a shake-up.

And either version of "potential pawn promotion", with or without the "potential queen" variant can be applied to large board variants like Capablanca Chess (http://en.wikipedia.org/wiki/Capablanca_chess) (which includes the rook+knight and bishop+knight in the starting array), or to Fischer Random Chess (http://en.wikipedia.org/wiki/Chess960), or really just about any chess variant that still closely resembles modern standard chess.