After the initial week of the campaign I'm looking into adding some of the 40K expansions into the mix. These supplements add new levels to the game and allow for interesting choices. While not necessarily the best for pick-up games, planning ahead for their inclusion into a campaign is the best way to use them in my opinion. That said let's look at what each of the expansions has to offer.
Cities of Death
Released near the end of 4th Edition the Cities of Death expansion revisited and revised the original City Fight ruleset. This expansion also brought stratagems into the game. Stratagems allowed players to give units or buildings special rules to reflect the special forces aspect of fighting in an urban environment. This expansion also modified the way buildings work (and was subsequently incorporated into the 5th Edition ruleset). For campaign games this ruleset is a great way to contest urban environments and makes for a very different style of play than a typical "open field" style game.
The big release a few summers ago, this expansion saw the release of special terrain elements in the form of numerous plastic kits. Playing a Planet Strike game requires quite a bit of prior planning on the part of both players, as your force organization changes dramatically depending on if you are the attacker or defender. Also special terrain such as fortifications, craters and defense lines are a necessity. For campaign games this ruleset can be used to simulate boarding assaults as well as planetary invasions. Given the nature of this style game players will need to be notified well in advance so they can prepare.
The vehicle expansion was released in White Dwarf over the summer and a pdf of the ruleset is available for free on Games Workshop's site. Like the other expansions this ruleset dramatically changes the way the game is played making tanks and squadrons scoring units. You also play using length of the board rather than the width. While this seems like a fun thing to try, not every player has access to a lot of tanks and as such requires much planning to set up a game. For campaigns this is a great way to simulate a force charging across the wastes to assault a position. Again given the nature of this type of game players need to be notified well in advance so they can prepare.
This expansion removes the force organization chart so players can use any model in their collection as well as super heavies. It requires a larger table and takes a relatively longer period of time to play a game. Playing a game requires a significant amount of planning, space and time. For campaigns this is typically the final game allowing all players to field what they want and all play together. Of course if their are titans or other super heavies on the board, you may spend most of your game setting up models only to remove large swathes of them before you even get to move.
Volumes 9 and 10 provide the meat for my current campaign. Not only do they include special characters and army lists, but also a well thought out campaign system. Also included in volume 9 are rules for boarding actions. This is a cool mini expansion that adds stratagems, and special rules to simulate fighting onboard starships.