My game: Invasion (working title): Dice Mechanics (long)

[Spooky Fanboy]

Do you see this as a mission-based (Shadowrunesque) game - where the PCs have a specific mission to, say, take over the police station?

If so, what happens when they accomplish said mission? Do they just stay there using those bodies as puppets forever? That seems anticlimatic, so I hope not.

Had a thought on that:

Supposing the aliens are viral in form and in methods of reproduction. So, they "infect" their target host. To what end? To leave a (nearly) exact duplicate of themselves, complete with all memories acquired up to that point, inside the host, and move on. It is assumed that PC(a) will remain in touch with PC(b), and stick to the original plan of the invasion.

But, after a time, PC(b) will no longer think exactly like PC(a), which will make things interesting if the host gets re-infected by a competing alien, and thus the memories also get digested and integrated. Further, there would be no obvious way of telling that there was a new infestation of the old host, so paranoia will often ensue.

And, of course, PC(b) or PC(c) may develop different ideas about the takeover of the human race than PC(a) maintains, so it is possible for them to change their minds without being taken over.

Also, what if a human gets "infected"...and his/her immune system wins? Let's assume, for fun's sake, that the human gets strange dreams, dreams of an alien species possessing people, dreams which seem damn vivid...and starts pursuing it, only to find out the truth! Further, the human in question will be near-impossible to "infect" because now his/her body is immune to the aliens in question!

Just some ideas to spice up the game...