@Ourobouros: Judging from past precedent, don't expect the new rulebook to solve all of the current problems with 5th edition. Instead you should count on at least as many new problems as problems solved.
@MarikLaw: Don't fooled by the Imperial galactic map. Barely one tenth of the worlds in the regions shown are actually Imperium-controlled. This is explained in the current rulebook iirc.
edit: Nothing happens on the 40k timeline because 40k is a setting, not a developing narrative. Of course GW could surprise us on this but I expect it to influence model sales and little else.
For example, the current rulebook background says there are more Eldar following the Path of the Warrior.( Phil Kelly emphasized Aspect Warriors over Guardians in the 4th ed. Eldar codex.) Result: GW sells more Aspect Warrior models.
Another example: In the 5th ed. Space Marine codex, various chapters train veterans as Sternguard in response to the desperate, dire times mentioned in the rulebook background. Result: GW probably sells more Sternguard models.