I am all for this. This is something that the Lindens can do where they don't have to worry about stepping on anyone commercially, and it can encourage growth in the surrounding areas.
There is a multitude of existing Linden Land in every public sim. All of that can be turned into greenbelts, instead of the wasteland that most of it is now. And what about the open areas around Oak Grove, etc? Those could be turned into something like a Central or Golden Gate Park. Even the void sims could be turned into something that would be nice to be around without using up anywhere near the number of prims to cause the servers any pain.
It will take a lot of mind changing by the Lindens to do this, though, as even something as important to our survival as an inviting Welcome Area has been turned into a commercial center.
Those of us with money to burn are trying our best to provide open, natural areas that folks can enjoy without clubs/malls, etc. However, we still have other building and creative needs that need to be fulfilled. On Parrot Island, our ground area is mostly open space with few structures and a lot of water. We still want to build other things, though, so we have created a platform in the sky for our other projects. It is a compromise, to be sure, but it is one that is working for us. It could work on the mainland as well, as Alex pointed out.
I do hope that LL decides to do something like this, as their creations in the new continent have shown a lot of pent up creativity. This would be a great outlet for their talent.
In the meantime, I do think that we have to keep doing this ourselves. The Park Registry is a great asset to the community and is a resource we need to keep using.