A Checkpoint mode option which creates checkpoints every time you open a door with a key
It could be interesting to have a Checkpoint mode option, which creates checkpoints every time you open a door with a key (checkpoint mode could be off by default). Once you die you could be prompted by a message which tells you "restart from last checkpoint" next to "try again" and "exit level". If you haven't yet reached a checkpoint after you die, then make "restart from last checkpoint" behave just the same as the "try again" option by restarting at the beginning of the level.