I have been involved in activist organizations (not Physics related), so I know a little about encouraging participation. PO has a bit of problem, in practice, broadening activity to encompass all of higher level Physics. By self description, PO welcomes building up latent categories, but what intentional action has taken place towards this goal?
As individuals with interests outside the usual show up and see little relevant to them, they go away. Perhaps you are fine with this. New sub-communities realistically do not build from 0 members to 1 ... then 2 ... then 3 or more. They need to nucleate, go directly from 0 to 3, 4, or 5. Which means a coordinated immigration of a small group of professionals or students.
The culture here is tightly identified with Theoretical Physics, Mathematical Physics, and High Energy Physics. Nothing wrong with that if this was Theoretical Physics Overflow. Assuming PO is serious about expand scope in practice, not just in principle, one of two things have to happen.
1. PO goes out and recruits a seed group for a neglected category. I would guess the probability of that happening to be zero.
2. Welcome a distinctly different group of new participants. This could be problematic however. They would, almost by definition, not fit in. Their field of study would be little more than superficially appreciated by the current membership, and vice versa. Probably considered a lesser endeavor. And only partially culturally miscible with the current substrate. Possibly considered invasive, since inevitably their inclusion will mean change away from what has become customary. They will not be Theoretical Physicists who also do X. They will be X Physicists.
I might be able to make this happen, but I am starting by scoping out the politics here first, so I don't waste anyone's time.
Related discussion: Would a site w/ all the features (eg, Comm Peer Rev) of Ph Overflow, but focused on all of Physics, be useful to you personally?