Add like, huge support columns everywhere in a few places, and hang glowstone from the ceilings of the big cave.
I dunno what else to say, maybe a minecart transit system.
something with redstone would be fun, I'm just starting to get into that
There was a map just like that a long time ago. Don't remember the name.
Back in Indev, there was a map type that was flat, but spawned caves, trees, lakes, etc. And you could dig down to normal bedrock level aswell.