I don't know why you guys are so stuck on cloning and resurrection. Starkiller either died on the Death Star, or Proxy did. I know Marek is not one for fear. Here are two possibilities:
1. The Netherworld is a huge place, much larger than people can think. Its the heaven of Star Wars people, come on. So, if Marek really did die, then that would be the netherworld. Because, I bet that developers want to give people a look on what the Netherworld is. Pretty much, it is what you want it to be, like the Nexus in Star Trek Generations. Watch it sometime.
Picard "died" and got in the Nexus. In that movie, it was referenced you could go anywhere at anytime. Picard went with Kirk out of the Nexus. So basically, Picard called himself out of the Nexus, nobody called him back. So, it seems that the Netherworld is the Nexus of Star Wars. So, I think that trailer had a few references to that movie. Visually.
2. After Proxy was destroyed, Galen trained himself with Yoda. Yoda speaking to him gives it away. Also, that cave on Dagobah has some power, so If this was the case, then that cave changed surroundings to an arena, like in the trailer.
So yeah, thats my 2 cents
Again, I highly doubt this game will use cloning. The whole concept has been done to death.