Personally, I would predict either June or maybe July (Augustus is a dead month, release-wise, for obvious reasons). The Rome II team is probably the only truly separate team, whose members don't regularly join other projects, depending on where priorities lie. In what concerns the rest, I am certain that their structure is somewhat flexible, apart from the core team, with unit designers and etc. frequently moving around. What I suspect happened with Rome II is that CA noticed that the last major historical game remains playable, while many customers are thirsty for history. If you remember, because many bizarrely enough mistook the pyramids of the lizards for some sort of Mayan ruins, they got quite disappointed at the Warhammer II announcement, which forced CA to adjust its business policy.
Presumably, releasing minor DLCs for Rome II is a nice way to make money and please a noteworthy portion of your fandom. It's not a coincidence that production is centered in
Bulgaria, the country with the lowest minimum wage in the European Union, even in comparison to Lithuania, Latvia or Romania. The low salaries and cheap prices allow them to generate easily profitable DLCs, with tiny expenses. At least until the 3 Kingdoms (when the number of Rome II players will inevitably fall dramatically, even if the release is a disaster), we should expect a constant production of downloadable material. It's a growing tendency in the video-game market, to invest more on DLCs than new titles or expansions, because it's far more profitable. CA's strategy seems sensible, but I wouldn't expect anything groundbreaking. In fact, the success of Empire Divided may encourage them to avoid an ambitious project like Alexander, but yet, the Macedonian conqueror is so popular, that in the end, I think a relevant campaign will appear. Perhaps, even the easternmost factions in the main game will be appropriately updated with Indian names, culture and units.