Hello! For non-canonical tags, AO3 defaults to the capitalization from the very first use of that tag and we can't change it, even if we try to type out the capitalized version. If we were to try to add "Dean Winchester (Supernatural)" to the tagset, it would automatically change to all lowercase. However, we can swap it with something like "Dean Winchester (Supernatural TV)" and that capitalization should stick. Since it appears nobody has offered or requested the tag yet, I'll just do that right now.