This sounds like a really good idea.
I would also vote to make it a new thread, while leaving Book 5 open. People who are not currently writing their characters would not make the transition, I think. Such as Jennabard. This would simplify our story considerably. We could call it
The Order of the Dark Flames Book 6: New Beginnings.
I could see us going back about 500 to 1000 years in the past (before the Time of Troubles) to Ogre Falls, back when it was still an active fishing village plagued by occasional marauding bands of Ogres (anyone see the Magnificent Seven lately?

) The portal might transport us back with amnesia of the events of the last few years, but with some training still intact. We could all be around level 4 or so. The transformation might also make those of use who are middle-aged a bit younger. Okay, only Gwally falls into this category.
The transforming effects of the portal could also cause Scayde to become a Faerunian rather than an Earthling if she wants. It up to her. Would you like to write a Ranger character or something, @Scayde?
Chan could manifest as a younger version of himself, or the portal could transform him into something else entirely – human perhaps? Centaur? This story can be whatever we want it to be.
For Gwally, this would be before he ever met his (ex) wife, and when he was still learning to be a Druid. He could react to everyone and everything as new and different. I see it as a lighter story line where we can inject more of the humor into our characters and stories.
Maybe Gwally has never met an elf before? Imagine the questions he would have for Aegis. Maybe he wants to know what it is like to be a mage? He could quiz DW. How will he react when he sees his first hot tub filled with naked women?
Eventually, once everyone is ready to go back and finish the Abomination story, the portal could re-open, depositing up right back where we left off – just the way we were. I would see it as Book 5 going into a hiatus, while Book 6 becomes an extended interlude. Book 6 would give us an opportunity to explore lower-level characters, perhaps even different characters, without the current weight of past baggage and trauma.