Have you read The Next Mainstream Programming Language? The original PDF was written by Tim Sweeney, the founder of Epic, the video game company that created Unreal. He really got me thinking about the problems he was facing.
Let's suppose you have 10,000-100,000 objects. Let's suppose that at any moment any one of them might need to "touch" 50 other objects. That's the setup.
Now, let's suppose that you also have a super-lightweight threading system that can handle 100,000 threads. Yeah, sounds far-fetched, but you'd be surprised. Consider allocating a thread for each object. Along with its own thread, each object should have an input queue into which you can post new events. Naturally, this input queue should be protected by a mutex.
The interesting result is that because there are so many threads trying to access so many mutexes, the chances of any two threads trying to access the same mutex to push a new event is relatively small is you assume a random distribution of events. Practically speaking, more fine grained locking can result in reduced thread contention.
Of course, life isn't random, but it's an interesting thought.