According to this news story from IGN.com, Level 5 (developers of the Dark Cloud series) would “seriously consider” developing a 3rd Dark Cloud game.
Dark Cloud 3 development under “serious consideration”
If this story is true, the key factor in determining future development would be public demand. Level 5 is a Japanese company, so unless you understand Japanese – it may be difficult to contact them. I, however, will provide a link to their translated contact page here.
Here is also a petition for Level 5 to develop this game.
