How about we name it hardcorejellyfishbutreallyghastfirelavascarynothelloblivionnehterworldcenteroftheearthveryscaryevilerthana creeperland. or we could just leave it as hell, as everyone will call it. Just my 2 cents.
The Origin will be the place monster spawner blocks get their "magic" from, so they will be a place that act as a gateway, but they're caged in so they don't act like portals. Only like a one way thing.
I like underworld. Its classy, like old-timey rpg with the Overworld map and the Underworld map.
I really like "underworld" and "overworld". They are generic enough to work with all the other generic stuff we have. We don't have any specific names yet...and I think a name like the Nexus is kind of specific.
EDIT: Oh and tesseract is completely retarded, almost as much as some people honestly confusing the word rainforest for anything else. Yes, minecraft is a game composed of cubes...but that is the art style and has little to do with the actual content of the game. WoW didn't start naming areas "Cartoon World" because it had a stylized art style.