FNC mythology always toys around with perspective. You're never quite sure who is good, or who is evil, and it can change depending on who wrote the text you're reading and what their perspective is. We know that Bhunivelze dominates the mortal realm, but we don't know if that's actually good or bad. I assumed it was bad. But at the same time, he created Pulse and the fal'cie, which is kind of making all of existence as humans know it, so that's pretty good.
It seems like he's just interested in power. I think he wants the door open to the unseen realm because it's something he can't currently attain. It's hard to say, but if he's the focus of the third game, and it's a conclusion to the story, I think we'll learn a great deal in LR about it all.
"In the physical world, it contains within its form endless chaos. By the will of the deities, it gives birth to all living things. I speak of crystal. The eternal dream world of the crystal lies within the Unseen World. Even the gods long to find their way to that place. In all crystal, the heart that shines most brightly is called Bhunivelze."
Yeul's Confessions, Final Fantasy XIII-2
I assume this "eternal dreamworld of the crystal" is the "new world" Lightning is going to lead souls to. I don't know what else it could possibly be.
We really know next to nothing about it, or about crystal in FNC. The driving force behind the story up until now was trying to get to the unseen realm. Now it seems like hidden in the unseen realm is a crystal dream realm. And FNC means "Tale of the Crystal" so this could be the whole point of it all.
I guess we'll see soon enough! I know I find it really interesting, even if it's confusing sometimes.