I wonder if this is possible to say..add some monkey bars for kids, or else, on a lot that does not show up on the map? You know, these places sims can walk to or go, within their district?
Like the community garden in willow creek, for instance. I am interested in another place, but a lot like that. can we edit it?

If we could, we wouldn't all be complaining about the so few choices where we can build.

As far I know no one yet has yet found anyway to unlock them, and possibility they never will.

Oh actually, I just thought of these ones yesterday...I knew that not being able to add lots was something people most complained about. I built my park elsewhere.

You can add some new objects to those areas, but the solutions are far from perfect. Your best bet is portable (overrides) of existing objects, cinderellimouse has modded portable basketball hoops and slippy slides , for example.
Placing them is annoying though, you usually have to try several times and even then it is probably not going to be exactly like you wanted.

Some people use something called "set as head"-cheat, but I haven't tried it. It has some restrictions, some bigger objects will shrink for example, and you have to leave neighborhood and return, otherwise the object won't stay there. And apparently it will stay there forever, whereas portable objects can be moved anytime.

I doubt they are lots so that's the problem. There are normal build mode objects scattered around the worlds and many world objects can be modded to work in build mode.

The game has no interface for editing the open streets. Maxis seem to use an external tool to design the worlds (going by the old Windenberg screenshots).

Yeah, that first idea is a workable solution. But it would also be best, since going to the trouble of editing the XML for the object to be placeable, to also set up a working Live Drag function on the objects, so there is a lot more fine tuning in the player placing the objects from a sim inventory, rather than the sim plonking it down willy-nilly themselves. Though not 100% sure that would work on the public areas in question here, as Live Drag usually doesn't work when off lot. Then of course, totally delete the sim who placed it as if the sim never was (No other family either), so the game doesn't try to give the household back their "forgotten" objects. It's a theory.

The set as head trick changes the size of everything, some noticeably so, some only slightly. Though that's only a problem if the interactions are a direct and continual usage of the object, like a child's climbing frame. That'd look odd.