Whatever they did, the grimdark mood would remain the same, or maybe become even more grimdark.
People aren't looking for any huge changes, just something to move things on a bit. The 6th Ed book did this in very small and subtle ways, like saying the Golden Throne was failing and no one knew how to fix it, the astronomicon was flickering, leading to whole fleets getting lost in the warp, tomb worlds wakening, hive fleets closing on Terra.
At the time these things all raised debate and got people to re-engage with the universe, even if none of these things were properly explored in any other publication. So if they had just dropped in a few more easter eggs like these in the 7th Ed book, or expanded on the 6th Ed ones, people would be happy.
I'm not looking for anything like the massive upheaval that Nagash seems to have done to WFB(though I wouldn't mind if they did), just an idea that the whole thing is not stagnating. Some things are just going on way too long though, like Ghazkhull's Whaaag(more than 20 years), or the 13th Black Crusade(coming up on 20 years), and need to be wrapped up one way or the other.
Personally I think GW is too hung up in the whole 40,000 thing. So what if it moves on to 40,001? It reminds me of the whole Y2K fear. People stressed over it for ages in the build, and then it passed without a hitch. The same would happen if they did advance it a bit.
I have never played WFB in any serious sense, but there are a lot of guys I know who play at a very high level, representing Ireland at the ETC for years now, and they are all very excited at what Nagash has done for the system. It seems to have reinvigorated tired players, and brought old players back to the game. If something similar could do the same for 40k, I think it would be a welcome addition.
