What are the most well-known philosophical theories that never gained mainstream traction?

Although the question is self-explanatory I'd like to set some boundaries for the answers. I am only interested in theories that were published in some form in the last 400 years in a Western country (so I'm not counting ancient or non-Western theories) and I'd also like for the general premise of the theory to be recognized by the majority of philosophers working in that branch of philosophy. Needless to say, I am aware of the fact that there is no clear line between 'gaining' and 'not gaining' mainstream traction, so I'd like to add that I'm interested in theories that never were anything more than a minority or fringe idea, so philosophers like Kant and Husserl are out of the picture. Finally, while the nature of philosophy as we know it today reduces the chances of a theory becoming mainstream to a minimum, I am aware that there are plenty of philosophers whose ideas have influenced thousands if not millions of people. For an idea to be considered in the context of this question, it needs to be radical in its entirety, i.e. not a somewhat radical spin on a widely-accepted idea, as the latter still manages to gain some level of recognition and can become the starting point of a whole new branch of philosophy or school of thought.<br><br>Some examples of what I mean by this are panpsychism, neutral monism, epiphenomenalism, solipsism and emergentism.<br><br>I'm not a philosopher and this is basically just a curiosities thread so I'd appreciate any help I can get.