This though was some years ago…
While yes, it was not really used, we could give it a shot again! The nether could be the place for people to have their bases in, which would be a new landscape people could live in. And yes, farms will be used there, that’s no doubt, but we have a good amount of new players so maybe they’ll have more interest playing in the underworld! 
Some things underworld could bring:
- Build comps themed in the underworld
- Mini games players could come up with
- New landscape for builds & farms
- Warps in the underworld (don’t know if it’s possible)