Stef,
What is your affiliation with QP? I was at the last Rocky Bluff NA meeting, and have volunteered to be a part of the soon to be formed Friends of QP. Were you one of the other interested people?
Having lived near and ridden at the Park for a dozen or so years, I would offer that the park has two major selling points. One is that is is close. The other is that the city has been very laissez-faire about what gets built there.
The people who ride and work on the trails tend to be younger, and as a result, transient. This means that some years (this being one) the trail gets 'found' by some riders who do alot of work. Then they get a job or into school somewhere else, and it might be neglected for a couple years. Eventually, it gets found again. Because of its size, riders who aren't into jumping tend to use it as a ride of last resort, if at all, since in an hour you will have covered all the trail multiple times. It is a great place to ride with kids, though, because here is plenty of easy terrain and they tend not to be fast enough to get bored.
If I were to speak for all the folks who ride there, which might not be wise, I would say that we are wary of change. We work with many different land managers, and the freedom to design and build as one pleases that exists at QP is rare, but it seems to work here, so why mess with it? However, I have heard that the city is working on Master Plans for all the parks, and am not sure the current arrangement will survive that, so it is probably wise to consider some options. It is hard to imagine a realistic improved arrangement.
That said, I don't think there would be any resistance to most of the upgrades (pools, fire-pits, signage, art) you mention. Though I personally sort of like the unkempt wildness hidden right here in town.
To my knowledge there are no maps; it would be relatively easy to make one of the general loop, but the specifics sort of evolve over time depending on who is building and what is being neglected.
I have never had an unpleasant experience with another user, and my sense from the neighborhood meeting was that people thought the various users were getting along. Perhaps there have been issues I haven't heard of, but it seems to me that the terrain of the park causes people to self-segregate (what is fun on a bike isn't much fun to walk, and vice-versa) and that in the areas of overlap, one just needs to be aware and respectful, and people generally are.
My expectation for the Friends group is to foster increased understanding and interaction between the various users, and to have some organized clean-up/ weed-pulling/ brush-trimming outings, but perhaps I am underestimating what I have volunteered for; it would not be the first time.
I'll let someone else describe the ideal mountain bike trail.
I would be happy to meet at the park and talk sometime.
Jon