Fitness level already exists so the passive wl/wg system could be linked to that value.
What was discussed is a fitness center which counters the effect that DDD has, for individual charactes, including addiction, but also for the towns obesity rate.
I'd definitely like to add a system where girls try to lose weight in the background. It'll probably be part of the difficulty system as a feature that comes into play at standard difficulty or higher (with the current game being easy mode).
I'll include your suggestion that different personalities are more driven than others & that it be tied in with the corruption system!