As far as multiplayer is concerned, I think it's an excellent idea, and I've got quite a few mates who'd love it as much as I would; but I do think it would require a seperate set of levels, possibly characters, much like Portal 2.*
I like the idea of the 'jittery' possessed Sligs. That would really add atmosphere and immersion!
I also like the time-trial idea, with the ghost player from a previous playthrough or some such thing.
Further levels through DLC etc. (SLIGSTORM PLX) would add to the longevity of the game, too!*
*You wouldn't have to worry much about making these levels if you included an easy-to-use and highly interactive level editor of some kind. Eh? EH? You know it has to happen
