I was actually thinking a lot about this recently. The way I would work it is this:
Outposts (IE what now becomes cities after x number of turns) are now called homesteads.
Explorers can now spend an expedition point to build an Outpost after an appropriate tech is researched. Outposts work like Civ V forts in that they provide a defensive bonus to units on that tile. They also project a zone of control like units do vs hostile units (IE units can only move one tile if moving through the zone of control).
Unlike Civ V forts, they also claim territory on the fort tile and the directly adjacent tiles may be purchased with energy. Outposts also extend a satellite network in a 2 tile radius (ie, the fort tile and all surrounding tiles.
Outposts cost 2 energy per turn to maintain, plus 1 energy for every tile away from your borders. This should prevent spamming outposts far outside your borders on every possible strategic resource.
Improved strategic resources are added to the Colony's stockpile as per normal. However since Outposts do not have population, no tile can be worked.
So, these outposts serve two purposes: forts a la Civ V, and as a way to claim and use resources outside your city's boundaries without founding a whole city in an area that may not be worth it.
But there's more:
Homesteads founded on top of an Outpost get a head start depending on how many tiles surrounding the Outpost are already controlled. An outpost with every tile purchased means that the city is founded instantly. Cities founded on Outposts also get a medium sized production boost towards it's first building.
Additionally, there would be three upgrades possible for Outposts purchasable upon researching an appropriate technology. These would increase the maintenance by 2 energy.
Firebase: The Outpost becomes a Firebase, doubling the defensive bonus and orbital network, and providing the city healing boost to units on and around the Firebase.
Research Post: The Outpost becomes a field Research Post, increasing the line of sight by 2, providing +1 science for each improved strategic resources, and +1 science for every alien and alien nest in it's LoS.
Trading Post: The Outpost becomes a Trading Post, which acts like a culture/production Station, and levels up in a similar manner.
If someone can mod this they have my full permission to use these ideas
