While I appreciate the academic angle you're approaching this from, I'd like to bring it back to the practical side of things. Virtual challenges can certainly be effective, but at the end of the day, it's all about the bike. Social identity theory and gamification have their place, but for many cyclists, the intrinsic motivation comes from the simple pleasure of riding.
As for specific design elements, I'd say keeping it simple and intuitive is key. Overcomplicating things with unnecessary features can be a turn-off. And while external accountability can be a motivator, I've found that setting personal goals and challenging myself is often more effective.
In the context of solo or isolated training environments, I'd recommend incorporating elements that mimic real-world riding conditions. Virtual climbs and descents, for example, can provide a more immersive experience and help push riders to improve their skills and endurance.
But again, it's important not to lose sight of the fact that cycling is a simple pleasure. Sometimes the best motivation is just getting out there and pedaling.