I wouldn't buy too much into anything Nathan Stewart is tweeting as he seems to take great glee from dropping hints and seeing people speculate rampantly.
That said, Eberron seems like a solid possibility and is on the short list of what I consider "likely" candidates.
If they really are going to put some form of product for two campaign settings, I would categorize them as follows:
Likely: Dark Sun, Eberron, Spelljammer, Planescape, Forgotten Realms
Unlikely, but possible: Nentir Vale, Dragonlance, Greyhawk, Mystara, new setting
Very unlikely: Birthright, Blackmoor, Council of Wyrms, and pretty much everything else
I personally think we're going to see all in the "likely" group in some form or fashion, be it merely PDFs or actual setting books. I was thinking we'll definitely see Planescape/Spelljammer as a merged setting (spelljammer ships sailing the Astral Sea) which opens the door to everything else. I think we'll see Dark Sun when they feel psionics are ready and Eberron in way, although it might simply be a PDF gazetteer and opened up in DM's Guild.
EDIT: I added FR. I'm a bit torn on the Realms, because on one hand it has been the default setting and has some published material, so you'd think they'd be ready to move on. On the other, it seems like a good idea to "complete" the Realms with a proper world book. Hmm...
Between this and Keith Baker saying he was under a deadline for a big project, I can only hope.
I really do believe it's likely the two settings will be Spelljammer and Eberron.