The set to follow Return to Ravnica will be Gatecrash, as announced at the Magic Panel at San Diego Comic Con by Mark Rosewater.
Unlike Ravnica block, Gatecrash will also be a large set—both sets will contain 5 of the original 10 guilds. Return to Ravnica will include: Azorius, Rakdos, Selesnya, Golgari, and Izzet. While Gatecrash will contain: Simic, Dimir, Boros, Orzhov, and Gruul.
Both Return to Ravnica and Gatecrash will be drafted separately as standalone sets—until the release of the third set, “Sinker” (which contains all 10 guilds), when all 3 will be drafted together akin to a standard-sized block.
Also of note: At the prerelease for both sets, each player will choose a guild. One of the six packs in the sealed pool will be a special “guild booster”, and contain a guild-specific prerelease promo card—which, for the first time, you can now play with in the prerelease itself!