My friends and I all decided that it would be a great change of pace to start up a 40k campaign. After partialy volunteering/being volunteered for the job I tried to get to work on it, but I just haven't come up with anything.

I'm trying to make an interesting story driven campaign that would involve tyranids, black legion, imperial guard, necrons, world eaters, and eldar (I always thought it odd that none of us play space marines). Basicaly, no ideas have come to me at all. So what I'm asking for is just some basic ideas for the campaign. I know that when I get an idea that I like, I can run with it easily.

Any and all help would be greatly appreciated.

just have it based on a planet in the imperium explains the imperial guard
world eaters and black legion have been given visions from the chaos gods about some nasty relics or something/ they're trying to thwart each other
eldar see that all this is going to lead to bad stuff so they show up to stop it
could be an ancient necron artifact that the chaos are looking for necrons are either there already or they show up to protect their stuff
tyranids kinds juct go where the hive fleet go so one seems to be on the rampage and ends up in system, just another thing for everyone else seems to deal with.

The Black Legion has an infamous sorcerer who's looking for a powerful artifact of Slannesh. During their searches they were forced to take sides against the minions of Khorne and the Lord of the World Eaters craves the head of the sorcerer and to deny him the Slanneshi prize. The Eldar know that the artifact will give the minions of Chaos a method of sacrificing Eldar souls to Slannesh in a way never thought possible and must stop him, lest Eldar have much more to fear from the traitor mon-keigh. The Farseer if the Eldar has cleverly hidden information for the Inquisition to find about the movements of the two Chaos fleets and have sent Imperial Guard to track and engage the forces. The Eldar are using them as a meat shield to defeat the Chaos Lord, and will then leave to let the other factions beat the shit out of each other.

This area, an area heavily tainted by the warp, was once alive with the workings of the C'tan. An ancient tomb world has arisen in light of all the life teeming in the sector and the warp taint surrounding it. They plan on engaging forces to trap them within a pocket of space, then turn on several devices that will sever this pocket of space from the Warp, negating all psychic activity and interstellar travel. The Lord who awoke is fitted with a special version of this technology and, as such, became invisible to the Farseers scrying.