solve any sequential game (currently only works with combinatorial games, or 2-player perfect-information sequential games)
This is arguably the most important.
Making sure your Game#possible_moves function guesses what the best moves are first
can save a lot of time on alpha-beta pruning and iterative deepening.
Use efficient bitboards - you can look at the examples for inspiration, but make sure your board representation is fast, and preferably doesn't need allocation.
A lot of the algorithms have been inspired by Pascal Pons's excellent blog and the general Chessprogramming wiki.