Early game: Basic mole people trading post. It wants one item that's available pre-oil, gives back ~4x the resource cost in either iron or copper ore, stops trading after a limit. This is for flavor and to make the special spot seem relevant to the player.
Late game: Advanced mole people trading post. This one wants a high tier item, such as blue circuit, science pack 3, speed module 3. In return you get a mixture of iron ore, copper ore, crude ore barrels and coal. This output is somewhat random and streaky, sometimes getting stuck on a single item for several minutes, but over a time period of hours it is guaranteed to be close to the same amount of each resource as the traded item cost. Or would have cost without productivity modules. HINT.
There's a cap for how much they'll buy here too, but this cap is replenished over time. The result is that once you have some productivity modules you trade power and assembler time for a permanent resource income. The productivity modules are a big investment, this encourages you to transport stuff between trading post and a main base instead of building a local production center around each one. They are limited by certain spots on the map to make you spread out, and gives a limit on this income. You need to spend a little effort on infrastructure for sorting the return goods which will be mildly interesting.
This doesn't solve the problem of having a late game resource sink... but it lets your factory continue to do something. And is more rewarding to set up than yet another mining outpost.
... maybe they could also want donations of some item every hour, which wouldn't give you anything in return but make them happy for a while. You could have a nice graph over how many trading posts are happy with you over time...
(ok, so trading has been suggested before, but I got to thinking about this earlier and like the combination of gameplay and feeling of WTF).
