Once all sets are out you can point at me and laugh for being wrong, but
Here’s what I think Return to Ravnica storyline will look like:
Return to Ravnica simply introduces us to the world after the Guildpact, much like Scars of Mirrodin.
Gatecrash features some huge plane-wide event - maybe with a malfunctioning plane portal, hence the merfolk and set name. I’d even go so far as to claim the Phyrexians could attack Ravnica, but seeing as they’re still working on fortifying their new homeplane and the high chance of a civil war between the red Phyrexians and the rest, I don’t think that’s likely.
I don’t think the Eldrazi will be the troublemakers either. Eldrazi or Phyrexian involvement could mean Nicol Bolas would be involved too and - wait a second. Didn’t they make Nicol Bolas out to be the icon of both M13 and Duels of the Planeswalkers? Oh ho ho, I’m on to you, WoTC! But no, I think it will be something new, like with what happened on Otaria.