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).
--------------------Just waiting for the mothership...