It is likely Treyarch is working on the next Call of Duty game, be it a main numbered entry or a spinoff. Treyarch has developed Call of Duty: United Offensive (PC) as Gray Matter, which merged into Treyarch; Call of Duty 2: Big Red One (PS2, Xbox, GCN); and Call of Duty 3. All of Treyarch's standalone Call of Duty games have been console-only.
In order to keep the successful series releasing on a yearly basis, publisher Activision juggled entries such that creator Infinity Ward worked on Call of Duty 4: Modern Combat while Treyarch worked on the prior game. Given that setup, it is plausible that Treyarch would now be working on Call of Duty 5 while Infinity Ward moves onto Call of Duty 6.
Given the language of Treyarch's posting, it is almost certain its next game is set during the ubiquitous world War II. Infinity Ward, however, has seen critical and commercial success with its new modern-day game, which may prompt Activision to keep new entries in that vein. In a Shacknews interview this year, lead designer Steve Fukoda said the studio had not decided whether it would retain the setting.