Yes there is going to be a single player for dof. Its called Kingdom wars, but it wont feature the storyline in it. Think of it like the total war campaign. To experince the storyline you have to play the MMO part as you get the story through doing the quests.
"90% of what is considered "impossible" is, in fact, possible. The other 10% will become possible with the passage of time & technology." -Hideo Kojima