here is what I suggest,

instead of refactoring, why do you try to come up with a new
collection libraries. This can be quite fun to start from a white page
and avoid to have Dictionary inheriting from Set.

  I will the idea of andreas about parallel dev when it
make sense. I also think that if we look at collection they work
quite well from a client point of view. This is why I'm not really  
excited
to change them. There other parts of the system that I would like to
see cleaner.

Stef