Rather than a flat rate of increase, I would love to have retirement be more 'dynamic'. In the NFL, most players will continue to play until they can't land a contract. As long as a player is being paid a salary and is relatively healthy, they should have an extremely low chance of retiring. Conversely, if a player of 'advanced years' sits in FA for an extended period of time, or suffers a major season-ending injury, their relative retirement chance should increase, with most players retiring after a season or two of being unemployed beyond the age of 30.