Mostly echoing smallbrownfrog and tryslora: something along the lines of highlight to read, even the option to not say but have that explicitly stated. I am not a fan of surprises myself and like to know what I am getting into ahead of time but putting that in a highlight to read for those who do want it seems better than a tag.

I'd actually be a bit wary about labeling/tagging the fics because even obvious things like "happy" endings often have a bit of wiggle room for interpretation. One example I can think of is a fic where the two characters are either mentally ill or just plain old batshit. They think they are happy but no sane person would really think so. So if you're going to have wide open ambiguous or up for interpretation type endings, it seems like you're actually taking away from the reader. I've read ambiguous endings that I found hopeful/positive while others might have seen it a bit more critically and vice versa.