I think both leggings and long johns are a tight fit, but you can get leggings that are 95% cotton or even merino wool (they ain't cheap though). My partner borrows mine once the weather turns nippy, and we both wear them underneath the trousers. The only real downside is that it's a bit annoying with unlined trousers (which in our case are all trousers), because the fabric catches on the leggings and doesn't drape down the leg the same way. Unless you really have an eye, it's not visible but I find it a sensory annoyance.