Progressive chess is a highly popular variant in OTB play but few game sites seem to offer it. The basic idea is that the number of moves a player can make each turn increases by one each turn, i.e. white initially has 1 move, black has 2, then white has 3 etc. Putting the opponent in check immediately ends one's turn, even if you have further moves left. (There is a variant in which one can only give check on the last move of one's turn.) A player in check must relieve check on the first move of their turn or they are checkmated. I think this would be a great addition to the site.