I guess one big problem about going back in time is that the Forgotten Realms has always had a metaplot tied closely to events in it's novel line. If you back track to, say, the Spellplague Era, do you have to honour all the current 4E timeline novels? It would be difficult to design with that restriction over your head. But at the same time, to go back in time and completely ignore novels ahead in the timeline that fans have invested in is a big ask. So like someone said, the Spellplaugue Era would probably be better done as a supplement like the Arcane Age boxed set.
I had no problem with the Spellplague but I didn't like the 100 year jump. But what is done is done and you have to move forward. I mean, look at the Dragonlance setting, it had the controversial Fifth Age era after the Chaos War that many fans hated. But then Margaret and Tracy came back onboard and tried to restore the "damage" done by releasing the War of Souls novels. The damage to Anaslon's landscape was (mostly) repaired and the gods returned and some semblance of the old setting was restored. Perhaps a similar approach could be taken to FR. You could have a mini world shaking event – perhaps awakened Priordials conspire to destroy Toril once and for all but the PCs thwart their efforts. In the process, however, Toril and Abeir realign and their lost parts return to each other. This might keep old time Realms fans somewhat happy. While new Realms fans can still have dragonborn and teiflings (trapped on Toril), interesting spellplague areas, earth motes etc. that still remain.
Indeed, why not keep Toril's connectivity to Abeir and release an Abeir campaign setting book! I remember the Dragonlance Time of the Dragon boxed set, detailing the continent of Taladas. It was a product under the Dragonlance brand but was almost like an entirely new campaign setting.
