According to Electronic Gaming Monthly (EGM), Square-Enix is supposed to unveil its next epic Massively Multiplayer Online RPG at E3 convention 2009 next June in Los Angeles. (Via FFXIII's official forums)
The original developer team of Final Fantasy XI Online will join in the next-generation MMORPG team as expected. They are trying to make a different MMORPG with Square Enix last MMORPG. Although there are many guess about which game will be change into online game, we still have no idea. Anyway, it might not become a failure as the name of Square-Enix.
"The next-generation MMO that we're working on is going to be different than Final Fantasy XI. And even though we have the same staff working on the project, we're not going to make something that'll be exactly like FFXI." Stated by Final Fantasy XI Producer Hiromichi Tanaka (EGM, issue 236, January 2009, page 28)
In the article James Mielke listed some suitable features that should be added into the next-gen MMORPG such as "Streaming data uploads", "More instanced battles", "Microtransactions", "24-hour monster spawns", "Fewer menus", etc. So, what will it look like? Let's cross our fingers and wait.

Related Stories: Three All-New FFXI Expansion Chapters in 2009!
FFXI Update Preview: A New Beginning
Final Fantasy XI Online Free Trial