I was thinking that an outpost would consume a population to build like a settler, allow you access work tiles and make basic improvements on them, but have population and not build districts or units.
Since I base most of my arguments and discussions on History (or hadn't anybody noticed that yet?) my criteria and aspects of an Outpost based on history would be:
1. Cannot build Districts or build Units larger than Scouts OR civilian units of any kind - all of those require more resources in Production and People than a Smaller-Than-City Outpost is going to have. Basically, if you were to form a Settler or even a Builder out of an Outpost, the Outpost disappears for lack of any remaining population. The requirements to train a bunch of Missionaries or Apostles or Gurus would also take up so much in resources that the Outpost couldn't feed itself, so none of that either.
2. Has no intrinsic defense. Sure, you can have Armed Homesteaders, who might fight off small raiding parties, but even in the gun-happy US expansion of the 19th century, when the regular military withdrew to fight the Civil War in 1861, the frontier in Texas moved back over 150 miles because of settlers running for safety in the face of Comanche raids - they felt, even if they always weren't - defenseless.
3. Can exploit specific resources - like the 'sugar colonies' in the Caribbean, the earliest Greek colonies in Spain exploiting mineral deposits, the Mining Towns in the American West (Deadwood, Dakota for a familiar example), but it takes other factors to make any of these grow into a city. Famously, in fact, many of the Gold/Silver Rush towns in the 19th century collapsed when the accessible gold or silver ran out, and remain, if anything, small or ghost towns today. Truckee, Nevada, was home to a huge 'Boom' and one of the largest Silver Strikes anywhere in the 1850s: it is NOT a major city even in Nevada today!
4. Setting up an 'Outpost/Colony" requires resources from the Homeland. And the further away they are, or the more different the local climate/terrain is from what's familiar Back Home, the more resources they require. And technology to overcome the differences. In the Greek expansion of colonies in the 8th - 6t centuries BCE, virtually all the new Greek colonies around the Mediterranean, Spain, Black Sea coasts were in spots very similar in terrain and climate to Greece. The technology to make a bunch of Greeks survive in, say, a tropical rain forest/jungle (down the African coast) or in a heavy-forested northern European environment simply didn't exist yet (or at least, not among the Greeks!). In extreme cases, Outposts/Colonies may require continuous resupply of people before they get settled: Jamestown, Virginia killed 75 - 90% of the Europeans who originally tried to settle there, and it was several generations before the 'Virginia Colony' was any kind of a going concern and started producing anything positive for England.
5. Outposts/Colonies can be taken over by another Civ - or even a City State. Examples are legion - many of the Greek colonies mentioned above became dependency of the Scythians, Celtic Gauls, or North African tribes and groups, the Dutch colony at New Amsterdam in North America was snapped up by the British, and the Russian colonies (little more than Trading Posts) on the Pacific Coast of North America were bought by the USA - as, of course, was the French 'Louisiana' colonial holding in central North America.
So, perhaps the answer is to treat an Outpost/Colony as a way to exploit a specific resource only, only potentially and with effort later being 'expandable' into a city.
In other words, an Outpost is simply a way of making a tile eligible for building a Mine, Plantation, Pasture, or other 'resource extracting' Improvement which makes that Resource available to the Homeland.
Perhaps a simple Builder Action:
3 Charges turns a tile into a tile controlled by your Civ, so that with another Charge you can build a MIne or other Improvement there. Other conditions being correct, you could even build, say, a Monastery there for a Religious boost.
IF the tile is the same as the majority of tiles within your 'regular' Civ borders, it takes only 2 Builder Charge - it would be much easier for a desert-based Mali to Colonize a desert tile than, say, Tundra-based Russia.
Note that by requiring 4 Charges to complete an Improvement in an Outpost, you either need 'improved' Builders or two Builders - establishing a 'colony' in unfamiliar climate/terrain early in the game would and should not be easy. Maintaining it early in the game with the usual swarms of Barbarians roaming the map would pretty much require a Military Unit sitting on top of it.
It should be possible to Fortify an Outpost/Colony, but that should take twice the Charges an ordinary Fort requires, because of the extreme difficulty of getting supplies, manpower, and building materals to the site. The stone fortifications at Ticonderoga and St Augustine look impressive in North America, but they are very minor Forts compared to similar constructions in Europe that weren't at the end of a long, long supply line.
