This Is My Island

Start a new game on an island the biters can't walk onto. When the map is created, the mod loads land until it knows where the enemy nests are, clears the ones that are too close, and paints a ring of water just inside the rest. Build your factory in peace, and landfill your way out when you're ready.

What it does

Loads the land first. Out to the largest island size you set, before the game starts, so it knows where the nests are.
Clears nests that are too close. Nests and worms nearer than the smallest island size (plus the water and the gap) are removed, so the island is never smaller than you asked for.
Draws the shoreline. The island is an oval that follows the nests: it stops short of them by a gap you choose, and the coast never juts more than about 10% past a smooth oval. The water is never less than 4 tiles wide, and its width and edges vary, so it reads as a shore, not a ruler line.
Keeps ore on your side where it can. The coast goes out around an ore patch to take it in, as far as the nests allow. If nests are too close to enclose a patch, it stops short of it instead, stepping inside the smallest island size if it has to, since a whole patch outside beats half a patch inside. Where that isn't enough, the water narrows to 4 tiles to slip past the ore, and only if even that fails does it cut through. Starting patches are always inside the smallest island.
Does it once. Nothing runs after the map is created, so there is no cost to your UPS.

Settings

Startup settings, so set them before you create the map (Mods > This Is My Island > Settings):

Smallest island: 200 tiles by default (100 to 400). Nests and worms closer than this, plus the water and the gap, are removed. The shore only comes closer to leave an ore patch outside whole.
Largest island: 400 tiles by default (150 to 1000). Where no nests are in the way, the island ends here. A bigger island means more land to generate when the game starts, which takes longer.
Gap between the water and the nearest nest: 40 tiles by default (16 to 100). Worms and spitters can shoot across water, so a wider gap keeps them further from your shore.
Water width: 12 tiles by default (4 to 40), on average.
Shape variance: 50% by default (0 to 100). Higher gives gentler bays and headlands. 0 gives the plainest oval the nests allow.

Good to know

New games only. Adding the mod to a save that's already running does nothing, so it can't paint over your base.
Only Nauvis, the starting planet. No island is made if the map has enemies turned off or Nauvis is peaceful (the map option, or a mod such as Peaceful Planets).
An island is always made when enemies are on, even if no nests are near, because expansion and pollution bring them eventually.
Biters can't swim, but worms and spitters shoot. Keep the gap wide if you build right at the shore.
Ore beside a nest may still be cut, rarely. If biters are close to an ore patch near the edge, even the narrow water may not fit between them, so the water cuts through it. Landfill rebuilds the land where you need it.
Leaving the island: landfill. It's the way out, and the way back for the biters, so build your defenses before you do.
Resources and cliffs under the water are removed, and enemy units already on the island's side are cleared.
Works with Space Age and without it.