According a rumour reported by VG247, Ubisoft have started work on Far Cry 3. The action will once again take place in Africa, and development is said to the lead by Josh Mosqueira, who prevously worked on Company of Heroes among other things at Relic Entertainment.
Whether or not the specifics are true, it's easy to see that Ubisoft would have another go with the Far Cry franchise, as the last game sold reasonably well.