We played a campaign early last year, still using 4th ed. (with some home brew rules, of course) and had a great time. There were 7 players: IG, Marines, Eldar, Ork, Chaos, Necron, and Tyrannid.
We developed rules on reinforcements and replacements, movement, installations, air power, off-board bombardment, fortification, hidden stuff, searching for hidden stuff, and much much more.
It was an Imperial world with the IG defending it. The IG just had to defend. Simple (or not). They could recruit more IG every turn. They had factories to pump out more vehicles and wargear. They were fortified.
The Necron started out very small, but subterranean; they had to be discovered and attacked before they got too big. Other people couldn't use certain assets against them (bombardment, aerial, etc.) in their lair.
The Eldar were hidden. They had a finite force (i.e. no reinforcements). Their goal was the destruction of the Necrons and Tyrannids, largely by playing off one against the other.
The Orks were largely feral. They had an off-world warboss and mechguy who had rallied them into a waaagh. They were very numerous, but largely unequipped. They had to win battles (for loot), capture factories, etc., in order to further equip their armies. Their goal was conquest.
The Tyrannids were a remnant of a crashed scout ship that was supposed to be preapring the way for a fleet. They had small hives that grew to larger ones, laid eggs, mutated, etc. (think awful green things). Their goal was destruction/consumption.
Chaos was interesting. They had a renegade sorcerer with a finite amount of chaos marines. They were also hidden, though without shielding like the Eldar had. They had to raid and win battles and sacrifice captives for Daemon troops or upgrades, or reputation to attract further HQ types with more marines. By conquering imperial areas, they could recruit heretics.
The Marines were the only faction with a spaceship---a strike cruiser. They had a finite number of troops. They could drop pod from their strike cruiser. Their goal was for the IG to win.
We had all kinds of rules for locating our opponents, moving about a strategic map (which was province-based, not hexagons), and having battles. The most interesting part was that of using the different scenarios. Different kinds of contact meant playing different scenarios. For example, when one side decided to retreat before combat, we played a (what are those scnearios with 400 pts per side called?).
Probably the most fun was our kill team scenarios. Yeah, you could send in an attack force to capture that city, but you could also send in a kill-team to take out some bunkers before hand. Or the kill-teams could take out shields, assassinate HQs, etc. Needless to say, we had to make up more rules about kill-teams.
Anyway, I think the point is that it was the decision-making of what kind of battles we were preparing for that was the most fun. There were relatively few pitched battles compared to the rest. I bet we played 10 kill team scenarios per strategic turn, while a pitched battle only occurred every few turns.
I could dig out our rules if you'd like, but its fun drawing them up. All 7 of us had input. Drawing up the whole campaign was almost as fun as running it. We played until there was nothinng left but Imperials and Tyrannids. They had both grown so large that everyone could play lots of either.
That's a major factor you have to consider when drawing up your campaign. Beware it gets unwieldy. Make sure you have organizational resources in place beforehand to keep track of everything.
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