The likely pattern for Destiny's future will alternate between full games and expansions each year, according to a report from Kotaku. Interestingly, it's not publisher Activision that settled on it, but rather Bungie and its decision to introduce cosmetic items currently being sold through the in-game Eververse Trading Company. And it all goes back to the engine used to build the game.
According to alleged current and former Bungie employees speaking under anonymity, the developer tools used to create Destiny are not terribly efficient. One source gave an example of something as simple as moving a resource node two inches taking almost nine hours. While the original plan was to release the base game, two DLC packs, an expansion, and then two more DLC packs before a proper sequel, the workload simply didn't mesh with the engine, editor, and schedule.
That's why, according to one source, Bungie suggested they adopt the microtransaction model. "There was a bet that was, 'Hey if we did microtransactions, I bet you we could generate enough revenue to make up for the loss of DLCs,'" they told Kotaku. "Instead of it going Destiny, DLC1, DLC2, Comet [the codename for the game's expansion-sized DLC], DLC1, DLC2, they're actually just gonna go [big] release and then incremental release. So it'll just be Destiny, Comet, Destiny, Comet, every year. It’s basically just switching the game to an annual model."
Back in 2012, documents revealed Bungie cannot deviate from yearly content releases, thanks to its contract with publisher Activision. So no matter what, Destiny is going to be annual - it's just a matter of how much content and in what form those yearly releases will be.
The microtransactions are cosmetic-only (though debate within the community has shown how some view them as a slippery slope), and The Taken King revitalized the game in a far more significant way than either The Dark Below or House of Wolves did, partly because it felt like a direct response to player criticism of the base experience. If Destiny becomes a cycle of development that gives its creators more time to breathe and react to fan feedback, it may be for the better.