You could also tie some vital resource to a central location and make it diminish the farther the player gets from it: For instance, a mana stone that causes the player's spells to be weaker the farther they are away from it, or a microwave emitter that gives vehicles and gear less and less power beyond some range boundary.

You could also wrap the world if it's magical or supernatural, so that leaving east returns you to the west side of the map ala old 2d games.

You could actually base a whole game mechanic after this.

Have you ever played Journey?

Traditionally a combination of border types are used, for example any combination of:

• Water
• Cliffs (fall to your death)
• Mountains
• Lava
• Forcefields (better than an invisible wall)
• Impassable forest
• Fences/walls/buildings
• Instantly fatal opponents
• Game-specific arbitrary limits (e.g. collar blows your head off, game ends because you abandoned your quest, etc)
• Limited resources (e.g. theoretically unbounded but you run out of air/die of radiation poisoning/etc)
• Boringworld (can walk forever, nothing interesting will ever happen)

Islands are fine, but I personally like a combo both for variation sake and because I secretly believe I will find some way through. ;)

I'd like to add weather to that list.

• Snowstorms or dust storms that obscure player visibility.
• High winds that push the player back into the level.

You could also tie some vital resource to a central location and make it diminish the farther the player gets from it: For instance, a mana stone that causes the player's spells to be weaker the farther they are away from it, or a microwave emitter that gives vehicles and gear less and less power beyond some range boundary.

Interesting idea. That'd actually be an effective way of making the game more difficult the farther you travel outward, without unnaturally making monsters more powerful.

I like it because it potentially feels less like the designer is trying to punish you (for instance with a wall of million hit point Titans that you do one HP damage to) and the reason for the difficulty has is explicable and natural. You could extend it in a few different ways. For instance:

- If the recharge time of something is critical, it gets slower and slower the further you go from the central hub

- Same for anything that naturally regenerates, like health or ammunition (or the amount regenerated per cycle drops)

- A side game of risk and bravado could emerge, with players gambling how long they've survived in "the outlands." Could be especially challenging if loss is more permanent the farther you go (gear is harder to retrieve, or there's permadeath beyond a boundary).

A-10 Attack! from the 90's had endless wasteland with a soft boundary at its edges, so you could fly for an hour past the map border, then when you turn around you're just a few kilometers away from the edge.

Just look to Astroids.