Bohemia Interactive has laid out a roadmap of its plans for Arma 3 through the next two years. It all centers around three major pillars: platform updates, downloadable content, and a larger-scale expansion.
Platform updates will center around making the peripheral experience easier and more enjoyable. Finding and using mods will get revisions later this month, while a Game Launcher and Add-on Workshop will further refine those tools. A new tutorial "Bootcamp" is in the works as well, to give players an easier entry point.
Though the roadmap doesn't detail all of the DLC this far out, it does note quite a few ideas they have in the works. Helicopters will get a major mechanical update, and another "Marksmen" update is said to improve the authenticity of the firing mechanics. Those mechanical updates will be free so as to not split the player base, but the announcement implies that any new content will be paid.
Long-term, late 2015 will see the addition of a new expansion. It's so far out that Bohemia is vague on the details, but it hints at the possibilities of a new terrain type, revisions to the 3D scenario editor, and more civilian interaction.
The full roadmap is available on Bohemia's blog.