AGPO CONSPIRACY THEORY OVERDRIVE!!!!
Look at it this way - most of the 40k range is now plastic. There's not a lot of updating they can do for space marines, orcs etc in terms of range, and their policy of support means they can't just keep releasing new chapters so...
They've got to re-fluff it! A total change in the background bringing some of those lose threads to the fore, a new great crusade maybe, any excuse to totally reorganise human military structures. Ergo we may well see them doing something radical like bringing the Emperor or a primarch back. They can't kill off mankind and they can't really make them any more threatened wihout doing so. Their only option fluff wise is either to stop there (no more big campaigns/sales) or to have a fightback.
Far fetched I know, but hey, why not?
I suspect very little will change at the moment, but it does fuel the furnace for the future.
Julys WD has prophesy in it that mankind will fall and nearly be destroyed but will then rise to even greater glory than before. (the prophet was executed) It does suggest that a long way in the future they could change the whole way the galaxy feels.
They could kill off all the existing characters, some of the less popular SM chapters and alien races only to introduce completely new ones. There is mention of a species living among the ghoul stars that is hyper-violent (whatever that means) they could be introduced. Like AGPO said their could be a new emporer, or the old one gets resurrected and maybe put in an uber-dreadnought body and the great crusade begins anew.
GW COULD do all this and more. But i doubt they will to be honest
EDIT: or the emperor freed at last from his mortal bonds becomes a chaos god!!

tau forever!

The only problem the tau offer the tyranids is annoying gristle between the teeth. Even a lowly termagant can beat a shas 'o' in combat!
ooh, I like that! and factions are formed containing certain marine legions and IG battalions, whoa! they could even start developing their own technology, others might embrace the greater good and join the Tau to offer a pleasing side platter for the tyranids!!!
Edit: This pic

I think it would be cool if the Imperium's system collapsed. Humanity started reacting to the Tau and had a revolution, with new techs and new science, so humanity no longer felt like a dead-end in social progression. Trimming the human realms and making the new humans pioneers again would be awesome. I think earlier editions were more 'we're exploring and surviving in a dangerous galaxy' wheras now humanity controls most of it.
Also, the fluff for 40K is: if it moves, it will be fought. If it doesn't move, it shall be destroyed.
That is, except for the Tau, who seem to have some decent background, but not really stories as such...
Besides, at the end of it all everybody is dying except the 'nids... SM are losing gene seed, CSM don't have any, IG die in swarms and procduction rates of humans appears to be down, Eldar are dying out, DE are the same, Necrons are self-repairing, yet apparently falling apart (as a whole), Orks just are, they seem to be going neither way, the Tau are trying to expand, but everybody attacks them for the sake of it and the 'nids, if they felt like it, could devour the entire galaxy in a matter of seconds, but won't for NO reason WHATsoever... (entirely against their other background of: eat things to sustain itself...)
GWs Official preview
Pdf file for how it affects each army
Pdf file of new 40k quick reference sheet
Hope this helps!
Although not condoned by this site, the beta version of the rulebook is available to download on a torrent site. There is a link to it on samurai gunslinger if you are interested. (note the link is to the web site, not to the torrent-which this site in no way condones as it is not entirely legal!)