I think that you will see Oak Hill East back on the list quite soon.
I believe that the USGA and PGA of America work in concert. They certainly hold quiet summits, where they discuss which course the other wants. Each has a different direction and purpose. It is in the interest of the PGAoA to continue the growth of the game through its professionals, so that it would take up the muni/true public access torch makes sense.
No matter what we treehousers think of potential sites, money and influence decide the day. Inverness and Oakland Hills WILL both host an Open, should they want it. Olympia Fields seems less likely to do so, unless the M & I mentioned above are in place.
My dark horse muni to make an appearance is George Wright in Boston. Work continues to be done there, but as I've never seen the space, others will have to speak to its potential.
My dark horse, public-access, course is not yet a course. I don't think that the USGA has given up on the Erin Hills-style experiment quite yet. After all, the PGA is slated to be played at the unbuilt, Gil Hanse course in Texas. If pride matters, the USGA will want to pull off a similar stunt.
The USGA might think that $400 is chump change, and that Pebble and The Dos are true public access. Some of you might feel the same way. Most do not.
HAS ANYONE answered the question about Merion having more room, other than the "buy up all the houses" notion?