It doesn't matter to me or not whether The Travellers have anything to do with: The Dark Dimension, Limbo, Hell, etc... I just happen to think that Katherine AND the other Stefan doppelganger were pulled into the same place that Markos had been located--due to a spell of some kind that The Travellers enabled (with the Stefan doppelganger, they could have used Stefan's & Elena's blood to pull off that little feat--perhaps linking him to Katherine in some fashion). That way when they performed the spell to get Markos back, they used the fact that they had two doppelgangers there with Markos in his "spirit cell" or "spirit prison"--which may have been the Dark Dimension or something else entirely--they created a hole in the barrier the witches made by undoing the spirit magic that held it up (just as they did on a massive scale later on with Elena and Stefan's blood) and yanked Markos through the Anchor (after overwhelming it).
Whatever they did to get Markos, even though it may have only been a small sliver or tiny crack in the barrier to keep The Other-Side separated from various other sections of the afterlife, the doppelgangers (more than likely Katherine and the Stefan doppelganger from Atlanta--and possibly other doppelgangers like Tatia who was IN The Other-Side and more than likely another Silas copy too...also in The Other-Side) created a permanent and growing "crack" in the barrier for The Other-Side (much like the spell Markos used in Mystic Falls that undid Spirit Magic, potentially globally if they had let it keep growing). Something akin to this happened to the Other-Side and there was NO WAY to stop it, but apparently the witches (as Bonnie's Gramms made it sound, she made a deal that may have saved Bonnie and potentially Damon at the last moment) may have had or been able to communicate with the witches outside The Other-Side (those located where the humans go, some werewolves have gone and so have many witches as well...more than likely the true afterlife; where the "Guardians" are...). As noted, even Markos was scared about going to whatever place he was headed off too... So he may have very well had knowledge of just what his spell to get out had connected The Other-Side to, and I'm MORE than willing to bet it ISN'T where Markos was being held; it is a place far, far worse--that is why the Dark Dimension makes sense (as it is a witches living nightmare).
Next season should quickly let us know. What happened to Bonnie and Damon--as they (at least it seems Bonnie) may have escaped the horrible fate at the end of that tunnel and since Damon held Bonnie's hand, he just may have been saved "accidentally"... We will also find out fairly fast who the next "big bad" is, and if The Traveller's spell hasn't been reversed...things will get VERY interesting if the place they need to go to solve the issue id in the middle of town... But, if a phantom appears--Dark Dimension. If a Kitsune appears, well the Dark Dimension could still be in play, but I'm not sure why they would pick it (as they are very powerful creatures and would fit in FAR better in a season of The Originals--a good one for somebody to date, maybe Hope when she is back...and the evil one trying to kill The Originals and Hope for stealing it's "love", etc...). It may even be people that got sucked into that void, who have come back through a "crack" in our reality (a crack that will get bigger, much like Silas dropping the veil of "The Other-Side"; this time reality has a real chance of ending up just like The Other-Side...this may happen if the phantoms show up too--it seems like one of the most likely events that will occur to be honest) and are absolutely "changed"--their powers mutilated and no longer powered by spirit magic, their souls corrupted and at odds with their originals selves...
I think they decided NOT to name the "big-nasty" for next season in this one, so they would have time to think about it. It makes sense. Plus, they had to figure out what to do exactly with Damon and Bonnie; bring Bonnie back as a super witch or possibly even as an Angel--something I didn't see coming, but I still doubt this. If anyone is coming back as something really odd, it will be Damon, even though he was my last pick--I still guessed that if he died it might be that the writers decided to make him the angel or guardian unlike the books--to switch things up (which may mean Stefan will commit an ultimate sacrifice in the end, to make sure Damon and Elena live happily ever after...). Plus, Damon actually WAS a very good and decent person before he became a vampire, so that must mean something--even after he became one, it was the behavior of certain vampires around him that made him utterly cruel and vicious...namely Stefan.