I'm always up for collaborations, but then I am a pretty narrow niche. I write letters to companies and people to see what kind of reply I can get. My letters are highly biographical and a sort of journal. About all I can think of to do is to guest post for someone. Ironically, my Substack is actual letters sent through the mail with actual postage, so you would think a "letters" feature would map great to mu Substack, but I'm not exactly sure how.