It would be very nice to be able to set up a custom non-rated game for specific chess situations. 

examine the following:

Perhaps you or the other player wants to know how they could have played out that last move differently? set up a clone of that last game from move #x and see how it would play out

You and an opponent want to see how a particular situation can be resolved, (like attempting a checkmate with 3 knights)

You want to play out a silly chess variant (all queens instead of pawns) because it would be fun to try. 

etc.. etc..

now this is already being done with "custom openings" when you start a new game, and you aready have an editor to create examples online, so I'm thinking it wouldn't be much of a technical leap to connect the two.

you could even put a button on there to "restart" (and the guy on the other side can accept) to try again.