Nah. It wouldn't be impossible for some serious egghead to potentially come up with a mod, probably rewriting terrain from scratch, but officially nah. They have already committed to instanced underground interiors and I don't see why they would half-and-half it at this point.

Just as much as I would like to see true underground caves I would also love it if it was possible to find water at different elevations, but I don't see it happening. Rip waterfalls. Rip frozen lakes.

It would be cool if we could place down our own dungeon entrance and then plan out and decorate the instance that it leads to, but also that probably wouldn't be official either because it sound OP as eff.