you should check out the Trailforks app. i imagine a lot of the trails (legal ones) will be posted up there, and you can compare elevation to distance to find steeper and steeper trails.
I wouldn't mind showing some of the trails I've been riding in the Weiser area. They're mostly motorcycle and ATV trails, and are certainly not buffed. Some are pretty steep.
Look at Wilson Creek Trails in the Owyhees. The China Ditch is my favorite. There is miles of trails out there. 500, or as it's called Bingo is a fun trail also.