I can see where you're coming from. I enjoyed the occasionally sense of danger that games like FFXI had.
The problems I see with this though is that if they weren't level synced then one of two things would happen: Only level 50s could participate because these encounters would all be at the level cap or level 50 characters would participate in low level events and destroy the feeling of danger you're going for.
Also if they are unavoidable I predict these events being more frustration then excitement. Imagine if every FATE you run by forced you to stop and participate. I don't think I would go minutes without being frustrated and annoyed at the system.