I don't think the comparison of the medical "diagnosis" to which you refer to legal opinions is a fair one.
This is the Internet. It's largely full of crap and everyone knows it. To paraphrase Gene Spafford:
[The Internet] is like a herd of performing elephants with diarrhea
-- massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind-boggling amounts of excrement when you least expect it.
But the distinction between a legal question and a medical diagnosis is where it applies. We don't answer legal questions. We also don't answer medical questions. I suspect that the reason there's not a closure reason specifying medical questions is because nobody actually asks them.
We're not talking about a question though, we're talking about answers. And that's where my Spafford quote comes in. The Internet is largely full of crap and everyone knows it. But it's generally up to the individual to differentiate the crap from what's valuable.
We aren't supposed to give legal advice as answers, obviously. But the reality is that we often do, to a greater or lesser degree. We phrases like "if it were me" or "in my opinion" or even go so far as to be more blunt and then end it with something "...regardless, you need to see a lawyer" or something similar.
The difference with medical opinions is that they are talking about a person rather than a situation and we need to be discussing situations because otherwise there's no wide applicability to a Q&A site. But like legal "answers" does anyone really take a diagnosis seriously? Is a questioner going to say "Wow, I didn't know I had that, I'm going to tell my HR department and get some accommodations"? Highly unlikely.
So the real question as I see it is "How do we deal with people offering medical diagnoses?" and the good news is that we already have ways and use them:
- Downvote the answer
- Comment about it so anyone reading the answer sees that the so-called diagnosis is challenged.
- Flag the answer as "Not an answer" or with enough rep, vote to delete it.
TL;DR: If I were going to change anything (assuming the language isn't there already) I'd specify something that clarifies that (like questions) it's not permissible to offer any answer that otherwise requires a license such as legal, medical and psychiatric.
"Of course that's just my opinion. I could be wrong" -- Dennis Miller