Personally, I do not see the death of the Astronomican posing more than a temporary nuisance for the forces of Chaos. Even if they would use it to navigate the Empyrean (which is possible, seeing as it essentially acts as their goal line, a huge psychic light calling "this way to Terra!"), we have to keep in mind that the Astronomican's effect may well be replicated. To me, psychic powers are not unique, and if said beacon acts like "a light in the Warp" that is strong enough to be seen from anywhere through the Warp, then this sounds suspiciously close to the light-like presence that any potentially psychic species has, with actual psykers shining much brighter. In theory, I could well imagine a powerful Daemon Prince or perhaps a Daemon Primarch replacing the Astronomican with his own corrupted presence.
As for a post-Imperial galaxy, contrary to most I think it could make for an interesting background. We simply have to accept that it would be a very unfamiliar one, which I think is both frightening as well as potentially exciting. For an example from another sci-fi setting, the new "Legacy" era of Star Wars features a very different setting that is still heavily influenced by what one may know from the movies or older books, but at the same time feels refreshingly new in that it features entirely new factions, or had existing factions evolve into something that one may have never expected.
I would predict that Chaos would play a much more important role in such a setting, however, and that it would be a very different game. I do not believe that a "Chaos Imperium" would exist for long without splintering into countless factions all at odds with each other. Chaos is inherently self-destructive in that it encourages strife rather than cooperation. The only way to bring multiple followers of Chaos together would be either by brutal force or by pointing everyone at a common enemy. With the Emperor's death, the latter would be gone, and the former would be impossible due to the sheer size of the current IoM. The result would be countless local warlords all with their own armies and strongholds and allegiances and alliances. And the best thing: This is just like their gods would want it.
So, I think campaigns in such a setting could be like a mixture of Black Crusade and Rogue Trader, where the players attempt to impress their NPC lord and master by fighting his or her enemies, all the while plotting a coup to someday take over. Alternatively, it could be like a twisted version of Dark Heresy where players are bounty hunters eradicating the last remnants of the forbidden cult of the dead Emperor; common people who grew up in a Chaos-dominated society but who were picked by an influential Chaos lord for their skills and talents.
And then of course there could be campaigns where the players instead play as members of a local resistance, be it humans or aliens, or even an alliance that, in the face of Chaos as everyone's enemy, transcends the xenophobic racism that has dominated interspecies relationships for millennia...
The potential is there. All you need is to do is figure out how to use it.
Edited by Lynata, 10 September 2013 - 06:38 PM.