Of or relating to troops that serve at sea as well as on land, specifically the U.S. Marine Corps."
Yeah, space is considered "sea", why then are space ships designated as "ships" (look at the word "spaceship", notice there is a "ship" in there, and ships commonly travel across seas).
Seas are also thought of as endless expanses of nothingness inbetween shores, and while technicially they aren't endless and they do ahve stuff in them (water), this definition also applies to outer space (an endless expanse of nothingness inbetween planets [shores]).
Any military unit includes "demonbusting" if the demons come to them. Anything that aggressivly attacks a military force is fair game to be "busted". After all, in the doom story, it was the marine security force and the diemos/phobos teleporter labs that were *attacked* - the demons came to them, they didnt go to the demons.
Yeah, ghosts are a good idea. When phased out any physical attack should go right thorugh them, but energy weapons could ahve a 50% of normal effect on them (rotating phase 90 degrees means you are translucent and cannot be interacted with physcially by solid or liquid objects, but physical energy could have a small effect on you... rotating phase 180 degrees removes you from this reality and puts you in the other one that exists in the opposite phase that ours does).
EDGE has the capability to do what that person did with dehacked (in edge, give something "stealth" [i belive], and they only go visible when they try to attack you... you can still hit them while invisible though).
It would only be fair for the ghost to be able to inhabit a dead corpse - so long as it has not sustained immobilizing structural damage (ie, if it has a hole the size of new york blown in it [rocket hitting a trooper], then it should be off limits unless the ghost is willing to repair the damage [not likely]). Some behavior modifiers include greater toughness, higher aggressiveness, greater speed, and the ability to leave their body at any time.
In addition, i think they sohuld also be able to make a bid at possessing the player's body too, it could make your view jump around (like you were cutting something with the chainsaw) and you might only ahve a certain aomunt of time before it comepletly takes over in which to rid yourself of it (humms... how would you get it out?, and how would you know how bad it's getting over time... perhaps the player's view should steadily turn blue then to black or sometihng). What eveil monster demon wouldn't want control of a combat powerhouse like the player character?