Maybe an ability to run the game at double speed on parts we've already completed would be good. So far everything is on floor 1. Maybe some room numbers to mark the different hp or level of mice and when we've beat that room number we can double speed those room numbers from then on.
Something like the boomerang from dragon quest that does less damage but hits multiple enemies since we're fighting multiple enemies at once. A whip or flail instead of a sword.