Not sure if its been suggested before or not, and if it has, please link me to the correct areas.
Disclaimer aside, my suggestion involves spicing up the minecraft underground, but to use only the vanilla blocks and items. By which I mean, for example:
- Finding a section of Nether biome deep underground, possibly close to bedrock. Or maybe even have most of the lower layer being more akin to the nether, since we've dug so deeply.
- Finding existing, or custom biomes that use the minecraft blocks. [Ardite, granite, marble, quartz, etc..] Or even finding a mushroom 'island' or jungle biome underground, in a pocket.
- Possibly even larger and more natural caves.
In any case, the reason why I'm talking in terms of Vanilla-only additions [I know there are mods that already exist that do these, especially ones that spice up the nether. I like them as well] is because I'm working on a Vanilla+ pack. Mostly for personal use but I could list the mods and such if people are interested. And i'm trying to add as little 'new' contents to minecraft as possible, while focusing on exploration rather than building. And while I do like the Underground/Nether+ mods that exist, I do not wish to add new items. and if I do have to add new items to minecraft, I am cherry-picking the mods that do so. [For example, a wearable backpack mod that adds only said backpack.]