Einstein-Cartan theory is an incredibly simple modification of GR and is intriguing most particularly in it's explanation of spin as being connected to space-time torsion. It's very straightforward to understand even in a classical GR model. It may be speculative but it forms the basis for Loop Quantum Gravity theories and since they've become so popular lately it seems like an appropriate avenue of thought.
http://www.reddit.com/r/science/comments/lt6d5/mass_of_the_universe_in_a_black_hole/
http://en.wikipedia.org/wiki/Einstein%E2%80%93Cartan_theory
