Pretty good idea :) I wouldn't want it put in now though, b/c we don't have anything to do yet! xDD Gotta wait a bit for the rest to catch up {Prey and such xD}
I don't support this idea at this moment.
Mainly because the multiplayer needs fixing and updates and is far from complete as I can see. I think the focus should stay on the Multiplayer part instead.
Another thing is that a singleplayer mode would take a long time to make when this time could been put on actions, preys general bug fixes and other nom for the game instead.
The singleplayer wouldn't last long and would probably end within a an hour a few hours. Then what? Nothing really, you've played it. Might aswell work on a map of yours or make your objects for later.
The game doesn't really need a singleplayer mode if you ask me. If you know you will be without internet access, you can stack up on other games that might have a proper single player campaign. Personal thought here.
I wouldn't mind one in FH though. But working on the MP part of the game would be better.