It seems like since the precedent has been set, FR must have a catastrophe to explain the rule changes. But honestly, I'd love for the timeline to be advanced 500-900 years, so as to allow a "starting over". Rebuild all that is great and lovable about the realms, axe what doesn't work, and not have to be handcuffed by previous events.
The time inbetween can be a dark ages for the FR. Knowledge is lost, cities are destroyed, gods rise and fall, etc. However, since this is all part of the dark age, very little history of it is known or understood (meaning don't explain it all away in novels, sourcebooks, etc.). Instead, 4E forgotten realms starts with the renaissance. Cities and towns are flourishing again, knowledge is being rediscovered, and there are plenty of dungeons and ruins from the dark age to explore. The world is thematically very similar to what veteran players know, but it has been parred down enough that new players can pick up the world and get lost in it just like we did when we first discovered it.