Awesome addition!
It is an interesting approach to opening training, although the way of selecting best moves may be flawed.
Firstly, are these positions generated from the lichess db, or a more definitive resource (such as all GM games in last 10 years freely available and updated weekly here:
http://theweekinchess.com/twic )? Choosing 'good' or 'best' opening moves from a collection of online bullet/blitz games does not seem as useful as choosing best moves by frequency as actually played by GMs.
Secondly, engine analysis alone (and its inherent horizon effect limitations) can show good moves as bad and bad moves as good depending on what engine depth is used... which is why a more thorough short and long-term positional analysis than CP difference (as assumed when GMs actually play the line) is needed.
Of course, GM use and frequency of the line can also be problematic (they may have played it frequently, but it is then 'refuted')... so it is not as simple as choosing the most frequently played lines from a GM chess db (as done usually online e.g.
http://www.chess.com/opening/eco/B22_Sicilian_Defense_Alapin_Variation )
I guess the bottom line is that - unlike with tactics - there are a lot of dynamic, static, short and long-term features of an opening position, so creating positions via CP can be problematic (although I am not sure what the best way would be). Maybe, simplest, is just let others/opening book curators do the work for you? Grab one of the many free opening books and generate positions from that? (then you know the positions have been vetted and are used by the very strongest engines) Or maybe ask one of the professional vendors if you can use their book?
http://www.hiarcs.com/chess-opening-book.htm - 12,681,453 positions as of December, all from top human games (if they let you use it online, I would personally donate the yearly fee :P)
That having been said!! :).....
Feature request: an ECO/opening label toggle so that we can just train the opening we want (for example A10 to train all of the English, or drill down to A17 to train the English Opening, Hedgehog defence)
Thanks as always for the addition and considering any of the above!