I have been thinking of ways that the game/environment could respond to user-based criteria. For example, a feature that exhibits this behavior already is the volcano lap-counter or the personal-best confetti.
Existing features examples that might function:
If a very large group goes by the wind-mill it could begin to speed up in response.
If a user generates some watt value, perhaps in London, spectators could cheer louder or yell your name or other user-preferred phrases.
I am making a metaphor between Zwift and a dark ride, a term for amusement park ‘rides’ that are slowly going away in the States. They typically work like haunted houses but you ride through. Zwift is actually somewhat similar.
The features in Zwift can be even smarter based on other information. Such as proximity to a ride-specific goals or overall in-game accomplishments. These effects can be turned on/increased/decreased/off.
Other variables to trigger behavior: heart rate, cadence, speed, watts, workout/race/group mode, gender, xp or even equipment or other avatar-style-related content. I think another nice tie-in would be heart-rate integration in workout. So instead of pure watt value, you could get Z1-5 adaptation of ERG mode in workouts to optimize HR intensity.