start with an opening explorer. have a way to comment on each position, maybe even to up/downvote comments so that a user can give a good explanation of what is going on in the position and have it surface to the top. solicit comments for each move and position, prioritizing the most common-occuring ones. good, approachable explanations like this: http://www.eudesign.com/chessops/epin-01b.htm
let players upload their games. analyze them, show blunders, rating changes. nice commenting system to make analysis easier. take tactics from games and put them into a tactics trainer pool. use this to figure out how difficult each tactic is and to get users to classify them. use this info to help players understand what tactics they're missing in their games. automatically build their opening book based on what they play, and help them find holes
when using opening explorer, allow user to choose different game sets. ex: I want to know what people rated 1200-1400 play against this defense
take the chess position trai