Non-plywood subfloor options?

What subfloor options are there for solid wood or other natural materials?

I am planning an addition to my house. Somewhere in the range of 12x30ft. Everything I have ever done either used plywood or already had very old 150+ year old boards on it. Most of the 1x lumber I see these days doesn't seem strong enough for a subfloor. I supposed I could just line it with 2x4 or whatever dimension was cheapest. Are there options specifically for this?

What options have you heard about? What options are available near you?

Hey Mat.
So I’m guessing you have beams as a base?
The necessary thickness would depend on the distance of the beams. Also some insulation would be beneficial depending on your climate and the construction.

Here (dry, warm, southern Spain) we often build on the earth. In that case we like to use gravel, then a slab of limecrete with fiber, then whatever finished floor. Tiles or wooden floor.