Insomniac Games may no longer be Sony-only developers, they are still backing Sony platforms. It's a new Ratchet & Clank. At Gamescom in Germany, the studio just revealed Rachet & Clank: All 4 One.
According to the Insomniac Twitter feed, the game is "4-player online and same screen drop in drop out co-op coming to PS3 in Fall 2011."
Characters include Ratchet, Clank, Captain Qwark and even, according to our guys liveblogging Sony's Gamescom press conference, "those robo fish guys." Our livebloggers also report seeing a "four-player character cannon" and "giant vacuum cleaners [that] throw each other around the level.
Ratchet & Clank has mostly been a single-player franchise, though Insomniac did release a multiplaer-centric game called Ratchet Deadlocked late in the PlayStation 2's lifecycle. Deadlocked was focused on competitive multiplayer, not co-op.
According to the official PlayStation blog, the game has been in development at Insomniac's new North Carolina studio since January 2009.
Ratchet and Clank: All 4 One Coming to PS3 [PlayStation Blog]