I wouldn't mind a single player mode, but I wouldn't like being trapped with the annoying thought of me needing to complete a quest or being followed by enemies at some point and whatnot.
Where I live I don't have internet access AT ALL, so an offline mode would be excellent.
However, wouldn't it be much easier to just make it so that you can test your maps with characters YOU made, instead of a single feline? [I haven't played so I don't know if someone's mentioned it or if it's changed]
It would make it much more fun for it in that aspect. In honesty I don't see a single player to fit Feral Heart at all. If FH were to ever have an actual "Single Player" mode, there would have to be something more original and fitting to its art style. Since FH doesn't have any specific like rules game wise [like a society or a culture for the game's inhabitants], it wouldn't make sense to have missions at all. xD