Age of Empires Online is what you'll get... forever. Microsoft Studios announced that it's ceasing any further development on the title effective immediately due to finances. The studio will release a "small amount of content" in the near future, but following that, the game's features and civilizations will be set in stone as it moves into a support phase.
That doesn't mean the game's getting cancelled, however. The announcement emphatically states that Age of Empires Online will continue to operate as is, will have future community events, and it is not "dying."
The move from development to support was explained as follows: "Creating top-tier content, as we have been for the last year and a half, is very expensive -- too expensive to maintain for long, as it turns out. We can no longer afford to keep creating it. Age of Empires Online already has a very large amount of high-quality, hand-crafted entertainment, and adding more is no longer cost-effective."