For this idea, I'm suggesting that 3-4 regions of abandoned mainland are built out as forest on each mainland continent. A 'real life' example might be something like a national forest found in the US.
Each continent could have it's own theme for the forest (jungle, swamp, redwood, beachy?, etc.), and have trails/plants/flora/fauna.
Each forest would have a small rez zone (so you can rez a bike or a horse or something).
PROS
  1. You don't need to make any new regions. Claim 3 to 4 regions in areas that are mostly abandoned. Every mainland continent has spots like this.
  2. You add instant real estate value to all of those parcels near the new forest that didn't have any value before - a 'protected Linden Forest view' on the mainland would be awesome.
  3. Volunteers could build it. The precedent has already been laid for volunteers to build large scale events/projects in association with the Mole and Linden teams. I am confident that a call for volunteers would yield an amazing a list of mainland loving builders.
  4. Cost to build it would just be whatever time is needed for the Linden / Mole to keep an eye on how the builder team is doing.
CONS
I have no idea how you'd 'sell those parcels' near the forest in a fair way, but I'm sure there are lessons learned from 'the old days' about how to release new land for people to purchase etc.