I honestly thought that Old World Blues will be the next one, it all seems reasonable to leave the Burned man and Ulysses til last.
One thing no one seems to have though of: what if the burned Man IS Ulysses?! lol. seriously though it wouldn't surprise me if they did that...
Also, why is everyone so against area 51? It doesn't have to be (and IMO shouldn't) about aliens. In real life, it is most likely a weapons development facility. In game, it could be the same thing. The most obvious thing to do is to have it be the enclave (maybe we would find out that the enclave WAS established by the pre-war gov. and they have always secretly controlled area 51) control it. Or there could be some large group of people that have been there since b4 the war (remnants of the real US gov, or ghoulified scientists, or descendants in a situation similar to a vault) and they could all be doing some horrific experiments w/ mutants (some F.E.V. was perhaps give to them b4 the war? it would make sense. If it is a weapons development base, they would defiantly have stored some FEV there) and other monsters (similar to the enclave labs w/ deathclaws in FO3, except better, and doing more than just controlling them. Maybe a cyborg type deathclaw?). Hell, the end DLC boss could be some bad ass Deathclaw/mutant hybrid! Or they could get more creative and make some new massive monster, or some dragon type thing! After all, why keep using the same monsters (or slight variations of them) in every game (deathclaws, mutants, rad scorps, mole rats, roaches, ect. the only new thing was cadazors)? There's so much you could do w/ 51 its not even funny. They should get creative with it! The only problem is that if it's done right it will be very horro themed. I don't have a problem w/ that, but others might. Esp. since about half the DLC that's been made has a horror tone (Point lookout, dead money, the pitt).