Interesting idea Ebrien. Generally it seems that, at least in the later books, a place in the mortal world corresponds to a similarly related place in the Never Never. Curiously, it was implied at one time it doesn't mean that there is only one place on the other side.
So it could go to the Mother's cottage. It could also go to any related dark place.
The way Nicodemus spoke about it though suggests there is a specific place that people cross through, so not just any prison or dark place in the spirit world.
I wouldn't necessarily say it was Hell, even if Nicodemus was confident of his ground. In fact, I am rather surprised he chose a place where he could get locked up for an eternity. I also believe if Hell were on the other side of Demonreach then surely he should have been able to draw on that advantage against Dresden.
But I do think it is probably some awful place, perhaps another famous spirit world prison. I would hazard a guess at either Celtic or Welsh or Native American mythos.
I don't really see the relationship between the Mother's cottage and a prison for demons/monsters myself but it is possible. Unless you think that the Cottage or perhaps Winter itself is imprisoning some terrible beings.
Another place could be Hades' Underworld. I am sure more than one person has speculated it's Tartarus on the other side...which would be suitably awful.