Theories as to the complete ending....
SPOILERS
I think it's obvious that Bruce and Alfred didn't just Thelma and Louise themselves a la suicide pact. It's to create the illusion they're dead.
Now as to who was at the end. First guess is Batman himself. Bruce now loves exclusively as Batman.
My feeling is different. I think that the new Batman is Jason Todd. His story was left incomplete, he helps Batman at the last second and then disappears... If he is the new Batman, using the serum Scarecrow made then it would be about making amends.
Thoughts?