So I had an idea, been bumping around for a while. Basically, 'what's a good way to build a fort from the ground up without touching a pick?' You have no stone, but still want a fort. I've experimented with purely wooden forts, and they're very difficult to pull off. Trees just don't regrow in time to produce anything significant.
So the thought occurred, to use plants. I've debated how canvas walls might work out, and I've also wondered about using bone or hide as a material. I think the conclusion I've come to involves modding in one or two plants. The first is a wood tuber, or something. Basically bamboo, it's a long, thin, very tough plant. The second (debatable) would be a type of cloth, or perhaps a herb that produces an extracted liquid. Either way, you'd take the tuber plants, cover them in cloth, and then may or may not treat them with extracts for toughness. This would produce a block that could then be used in construction, or perhaps could produce a door or hatch as well, to keep the theme.
This would be somewhat intensive, taking 5-10 tubers and 2-3 cloth per block, plus whatever extract if that was included, but would have the benefit of industry. You could dedicate whole fields to this endeavor and produce a lot of material with enough workforce.
I also had the idea that a similar principle could apply to bones or leather. Leather + tubers is straightforward, but I was also thinking bones taken to a quern to produce bonemeal, then using cement weed to produce bone blocks. Especially people with modded ethics could make goblin bone walls.
Basically I've been concerned with how to do things without mining at all, or very little, as I'm fond of surface forts. But vanilla seems to lack legitimate surface options for a challenge like this.