First area, called Fortress Gaia.
According to a report put out by Stoo.com, Gaia is a large military facility that houses various areas where "battles will unfold." The base is said to contain stages set in a large forest and a city monorail, among other locations. What's more, the report also says Gaia will implement a differentiated play experience based on the player, whatever that's supposed to mean. In a nutshell, it sounds as if Gaia will act as the hub-world where players can meet up and embark on quests, taking place within stages. Educated guess right there, so don't take my word for it.
As for Duo, the article doesn't say specifically if he's playable or not, merely he'll be making an appearance alongside other key characters. Duo was chosen to appear in the game because, like X and Zero, is very popular among fans.
Finally, the report mentions that the Rockman Online teaser trailer released on the 26th of last month already has seen over 150,000 views, and received a huge response from not just South Korea, but Japan, America and Europe as well. Neowiz plans to continue releasing game screenshots, though I get the impression their idea of screenshots includes game artwork, mostly.
News credit: Protodude's Corner and TMMN.
With all of the views generated on that trailer, here's hoping that will help motivate the game's release outside of Korea.