Thought about doing a cart that was tall enough to take down some of our shitty gravel roads.
Getting the gearing right that would take some time to figure but would be fun.
My dad built me one as a kid that had a David Bradley gear reduced axle and Valiant spindles with steering box. It was not fast but would go through anything. You could still find a David Bradley for about 150 bucks to use if you like.