I know there are a lot of group Private threads that aren't just buddy threads and actually discuss topics that are too controversial or personal for the main boards. But there's no way of finding these threads to become a part of them.
Maybe we could have a stickied public thread where people can advertise their private thread topic, and members can then PT that person to be added. I know these threads are harder to moderate, but they aren't really any different to BTs.
Some examples could be help on a specific school subject (maths/ English etc), politics, mental health, relationships... I don't know if these exist, but obviously they can't easily be on the public boards, but may be ok for private.
If you are going to walk on thin ice...
...You might as well dance
I was once part of a PT for LGBTQ+ and I would have never have known about the PT if it wasn't brought up by one of my friends on here. Like you rightly said, a lot of people would love to be apart of discussions which meant a lot to them or something they perhaps needed advice on. Even to be there to give advice and support others.
I don't see a negative on this, either. If someone was wanting to be apart of the discussion PT, they'd have to PT the member to be added. And if they no longer wanted to be apart of it, it's easy to remove yourself.
Hopefully one of the Staff notice this and can bring it forward for discussion.
I think you would find this discussion really helpful for fleshing out this idea. The idea was proposed on page 32 and a lot of interesting questions were asked on pages 33 and 34, specifically on how this would affect the spam rules and which topics out of the current Topics Not Allowed would be allowed in it and which ones weren't.
I'm not really sure if the modmins have considered anything more since then, but that's a good place to build off of and presents some problems that'd have to be solved to see a thread like this one day.
I support this idea because I have a hard time finding any PTs at all to join, and a lot of us are older and that comes with new struggles and talking about things that wouldn't be considered so family-friendly. I'm already in the LGBT+ one but I'd really like to join a mental health PT or spirituality PT, for instance.