Suppose each player on the site has an opportunity to create a custom set of openings, and activate them all as pre-moves at the beginning of each game, such that two such players can "blitz out" their favorite opening moves and get to the meat of the game faster?
White leads e4, and activates all his standard book responses against 1...e6 1...e5 1...c6 1...d6, etc.
Black responds with 1...c6. Both players' books keep spitting Caro-Kann moves at each other until one book runs out, or one book plays a novelty.
For players who enjoy lengthy main-line openings, this would really save the trouble of having to make the first 12 or 13 moves manually or via conditional moves.
Yes, it's a lot like Deep Mode, but the difference is that these opening books would be persistent to every player's account, which would save the time of programming variations by hand for each game.