I'm in favour of a referrals-only forum, for one simple reason : if anyone with a bit of ppruning experience was to quickly think of a mental top 20 of the chief pain-in-the-ass thread-destroying flame-war-starting got-an-opinion-on-everything pompous windbags in this place, the majority of them are either current or retired airline pilots. Plus one disturbed Australian billionaire PPL and one Ryanair management lackey who masquerades as a pilot anyway.
Either way, requiring proof of a professional licence will keep away a few of the opinionated spotters and SLF, but none of the real trouble makers. Hence, this new serious-discussion forum is just as likely to go down the same pan of triviality and personal agenda as all the other existing forums, unless access is restricted ONLY to those who have already demonstrated both sufficient level of specialist knowledge and a basic maturity level in the forums in general. There are plenty of folk on here who have either one or the other but not both.
An invitation-only regime would also enable the genuinely knowledgeable engineering / ATC / cabin personnel to dispense their wisdom when it's called for, and I for one would be very glad to learn from them as well.