I was thinking this the whole time reading this thread. Now, this coming from someone that has lived 35 mins away for the last 12 years and just went the first time this past weekend. Why, have, I, NOT, done, THIS, before??? The funny thing is, I ran into a few people that said they haven't seen me there in a while???
I think we can get carried away with what "we" are looking for. Coming from BMX, we would go to a track and ride it for hours; practice berms and flat corners, clear that double, scrub those tabletops. My goto was in Pequannock because I could ride there. It wasn't big, but it had everything you needed. Gallons of sweat remain in that soil.
I think people hear "Bike Park" and expect Whistler Light or Ray's mini. But it can be really be mini, mini. There are a few secret jump lines in the area. The most comprehensive that I've seen has 3 jump lines that go from "oh that looks fun" to "fuck that, maybe next time." All hand-built in a very small area.
The point here is, start small. Going for it and developing the whole side of a mountain, well that's called MCBP. Somewhere between 50' and 100' of elevation is plenty.
I would say almost any park could handle this. An active MTB scene, the proper approvals and people willing to swing a shovel could get something done pretty swiftly.