As the OP requested, I'm looking at this from the standpoint of a Brand Manager, not what I like as my own personal favorite.
As much as the original Grey Box, and/or Ed's original vision might be my favorites, they've already been done. All the source material is already published and out there for those who want it (except of course the pdf's are unavailable
). So those would be out for me.
Having another World Shaking event ala the Time of Trouble or the Spellplague will just alienate even more fans (although I didn't have a problem with changes, I could just ignore and alter as needed, some were quite rabid about things changing). So, if it was my decision, no more World Shaking events (at least for the next couple of editions
).
Going back and doing a Spellplague Era campaign would be cool, but not as the big default campaign world. I'd do this more like the Netheril: Empire of Magic mini-campaign/box set from AD&D 2E.
So, that leaves me with what I feel is the most feasible, continue the 4E timeline. This has the least chance of more fan alienation, progresses the campaign enough where I can publish material that people will buy, and creates new npc's and world politics to exploit in adventures (modules and Dungeon Magazine).
Of course, my home FR game will continue to be a hybrid of the 4E timeline with 3E geography and my own houseruled D20 rules set.