1.) Slope Limitations is a Mistake,
and by extension
2.) Excluding Beacon Terraforming is a Mistake
Your inclusion of Painted Zones is the BEST ANSWER to opening Gamma AND reigning in out-of-control terraforming. We don't need to terraform and build on the entire island, just SMALL SECURE PORTIONS of it.
We don't need access to all the coastlines, and besides, why take a Shoreline or Peninsula defensive positions away from defenders? Why be forced to defend a base from 360 degrees? With a guaranteed open island it's fair to expect to find more secure, locked up portions within which to build a secure base.
Are you worried about Highways? We don't need to terraform highways as long as you design islands with passable terrain out-side Painted Zones (which should web and weave through the CENTER of the island as well as from its teleports). Forget structures and terraforming for Highways and implement your Speed Tiles to be placed on any passable terrain. It's SPEED that matters, not silly flat valley sidewalks. Perhaps even make Speed Tiles Visible and Usable only to Friendlies.
And what about Friendly Passable Shield Walls as opposed to "Gates"? Are we sword dwelling barbarians or futuristic sci-fi mech warriors? Do we have to "open" the gate or does a Shield Wall automatically block passage to all but Friendlies?
I wish I was a photoshop artist to create an example of my vision, but all I know are words.