Gas Powered Games has made the difficult decision to cease all development on Age of Empires Online, meaning what you see in the game today is pretty much all that you'll see forever. Citing financial reasons for the decision, the developer still has a "small amount" of content due to be released over the next few weeks, but no new features will be added and no new civilisations will be released.
Quick to point out that "nothing else changes for players", Kevin Perry, Executive Producer for the Age of Empires franchise at Microsoft Studios (writing as AOEO Trajan) stresses that nothing that currently exists will be removed.
You can still play everything you own, earn EP, and purchase any content you do not currently own. We will still fully maintain and support the game and its players; no one is losing anything that you have earned or bought. Our Community team will continue to support the game by way of Community Challenges, streams, contests, PvP tournaments and more.
In simple terms, "creating top-tier content ... is very expensive", and GPG and Microsoft Studios cannot afford to keep creating new, high-quality, hand-crafted entertainment, as they have already done for the past eighteen months.