While waiting for the scifi Chain Reaction (called 5150) to appear, I've been daydreaming about an idea I had for a campaign setting.

I was very impressed with the campaign work THW did with "All Things Zombie" and Nuts! (the WWII Chain Reaction), and it seems 5150 will have a similar setup.  Being somewhat of a 40k fan, the idea of an Tyrannid invasion of a planet seems like a very good setting for a solo or cooperative campaign.  Chain Reaction already has a nice set of "Bug" rules for playing Aliens/Starship Troopers sort of games, but they only deal with one sort of bug.

Tyrannids are conceptually similar to Aliens and SST bugs, though they have a great variety and some interesting hive-mind twists.  They also are known to invade a planet in a series of stages, from the initial scouting to the final devouring of all organic material.  So I've gotten it in my head to do a campaign game that plays out one squad or platoon's resistance to a Tyrannid Hive Fleet invasion.

This campaign will have a number of phases, and your success in one phase will influence things in the next phases.

First is the initial Genestealer invasion.  These nasty fellows infiltrate a planet and "infect" a colony of the locals, turning them into Genestealer hybrids.  Genestealers are smart and vicious, and try to work their way into the local society to rot it from the inside.  Once they have enough of a hive going, their psychic eminations will catch the attention of a Hive fleet.  In this way, the Genestealer infestation acts as an advanced scout for suitable worlds to conquer.

Players in this phase will be hunting down nests in urban settings.  There won't be many "alien" artifacts, and the human hybrids will use the same kinds of weapons that the player does.  The genestealers themselves will be the objective, and they're a nasty group to fight.  The hybrids won't put up much of a fight, but will have guns, so won't be harmless either.  (Rep 3, I'm thinking)

Next will be the hive fleet arrival.  The fleet drops "seed pods" which infect the very soil of the planet.  The infestation begins to "grow" a number of different kinds of warrior bugs to help secure the planet.

During this phase, players will have to fight swarms of bugs, but in mostly "normal" terrain.  Later in this phase, the enemy will be able to produce huge Carnifex "tank" bugs and other giant monstrosities that will be quite a challenge to hunt down and kill.

The final phase will be the evacuation.

Nobody survives a Tyrannid infestation once it gets beyond a certain pont, and players will be fighting in an alien landscape with dangerous terrain and huge beasts patrolling the countryside.  Swarms of rippers devour all organic matter, and the remaining warrior bugs strive to protect them from harm.

Most of the Tyrannid hive's effort at this stage is in stripping the planet, so players won't have as hard a fight, but they'll be trying to escort civilians to evacuation ships, and to protect those launch sites from Tyrannid beasts intent on keeping any organic material from escaping their maws.

Winning in earlier stages gives the player better equipment at later stages, and success in earlier stages can prolong the onset of later stages, but ultimately the planet is doomed, and the player only wins the campaign by helping to evacuate his homeworld.

I stll haven't worked out any details, but that's my basic concept.  I imagine it as a solo or cooperative game, with the Tyrannids being controled mainly by reaction tables and some "programmed" attack patterns.  The smaller bugs will be more or less just like the current Chain Reaction BUGS, but the smarter ones will have extra abilities, and will be able to psychically control the Bugs to make them more dangerous.