A campaign would be great, get some of the fluff-masters onto working out a background.
The campaign form being basically a more fluffy tournament, I'd prefer everyone to submit their say 1500pt army lists and then develop a nice story. Then we could post the results and what happened (and to who...) on here. The "LO" campaign.
We need to decide first - setting. I say Eastern Fringe simply because we might get Tau players popping up and that's the only place that makes sense for them.
We need to decide the campaign objective - a mighty Chaos Artefact? A lost Imperial Titan? Some ancient Eldar weapon from before the fall? (I particularly like this idea, as it would be a useful plot hook for the Necrons - think Winter Assault) Or alternatively some Necron installation?
For "sense" purposes, it would probably be best to explain the various Imperial Forces as part of a crusade, otherwise it is non-sensical that they would all happen to turn up at this same spot independantly.
Then we need to work out plot hooks for the various factions. Using the example of the "Eldar Weapon" (such as the Talismans of Vaul/Blackstone fortresses) e.g.:
Chaos presence: There to capture the artefact and corrupt it.
Imperial presence: There on crusade to destroy the gathering forces of Chaos.
Eldar: There to recapture their weapon against the Necrons.
Necrons: There to prevent the Eldar regaining their weapon.
Tau: Alarmed by the incursion of so many armies near their borders, and so investigating in force.
Tyranids: Splinter fleet attracted by the accumulation of biomatter.
Orks: They just like to fight
I'm sure one of the many worthy authors/fluff-meisters on the site will be able to write a narrative introduction, and then a narrative conclusion from the results.