What major problems will I run into if I ran an nWoD game with the classic WoD cosmology being the "true" cosmology? At this point, my major (only) concerns are the big three (Werewolf, Mage and Vampire).
Specifically, I'd like to keep the Werewolf triumvirate and the Mage cosmology as well as the classic Umbra. Having said this, I'm not married to the automatic Werewolf - Vampire enmity, I'd maybe downplay it to the usual fantasy Elf-Dwarf racial relationship. As for Vampires, I am honestly not sure what I want the real cosmology to be.
I'm willing to try a mixed supernatural game if my players want to and at least one of them wants to be a vampire, but that is my least preferred of the three. I believe that as of now it is the player that wants to be a vampire that has classic WoD experience which was almost exclusively VtM.
I have experience with classic WoD and am reading through nWoD currently. I like that more than just werewolves have multiple chargen choices to determine their characters' origin, though I suppose this will be somewhat problematic for mages if I ditch the Atlantis thing but keep the paths and orders of nWoD and the old cosmology.