Going back to the Age of Revolutions/Colonies idea... what would people think of them adding a new settlement type to support that idea?
Ideally in Exploration Age, both continents would be considered "Distant Lands" to the other one. This isn't he case as it is now, but I think Devs have hinted at fixing that and distributing treasures across both continents. [Got confused - they already patched in Homeland treasure resources]
That would then open the door to a mechanic where any settlement you create on Distant Lands is a colony, rather than a City or Town. If we want to go for simplicity's sake, maybe colonies are just towns that can't be upgraded? Otherwise it could get more complex, with different improvements/buildings associated with Colonies to try and extract treasure and/or serve military legacy goals.
In the Exploration age, that's all you get. Your colonies are explicitly to support the Homeland and you use them to win legacy paths. But then at the end of the era, a crisis happens where the Colonies rebel and you have to react in some way. The consequence is that you can either A) maintain your empire and consolidate in Modern, B) create a powerful ally to help you during Modern, or C) create a powerful enemy because you failed militarily and diplomatically.
These are kind of half-baked ideas, and of course not every Civ even wants to settle in Distant Lands. But at least it's a mechanic to create more players in Modern to shake things up?
Introducing a new settlement type just to provide 'colonies' is, IMHO, a bad idea.
First, because if you define a 'colony' as only being a settlement in a Distant Land, it makes the entire concept excessively Rigid - nobody can plant a colony of any kind anywhere else, even though Greeks, Phoenicians, Venetians, Genoans, Christian military orders, Austronesians, etc did just that for centuries. It also makes the entire Exploration Age a rigid 'plant colonies overseas' as it is now, which disregards numerous states and civs that did none of that IRL and proivides no alternative to the gamer stuck in he middle of a Homeland continent or in some other map-generated geographical backwater. Civ VII's loosening up of the Treasure Fleet/Resources in recent patches indicates that they recognize this as a problem, and I agree with them.
Second, because an arbitrary definition of a settlement as a colony is not necessary. Settlements can already be defined as Farming, Fishing, Mining, Trade, etc types. If a separate and distinct definition is needed for a colony, that can be added to the existing system. In fact, though, a 'colony' could simply be one of the existing types a suitable distance away from the nominal center (Capital) of your Civ with whatever 'special'; characterisics are tjought to be 'typically colonial'. For example, for those determined to recreate the entire European colonial experience in the New World, perhaps another type of settlement could be added: "Ruthlessly and genocidally exploit any natives handy."
The entire 'revolution' mechanic, which I completely agree is needed, does not have to depend on any colonies: note that three of the four most influential revolts/revolutions in European Modern and pre-modern history: the Dutch Revolt against Spain, the English Glorious Revolution against the Stuart monarchs, the American revolution and the French revolution, had little or nothing to do with any colonization at all: they were Internal and had to do with severe differences between what the government (Civ) wanted and what a large portion of their population wanted.
I would argue that a fundamental 'enabler' for much of this, and certainly for the last three 'revolutions' cited above, was the advent of Enlightenment ideology/ideas. - At least, that makes a clear and singular mechanic that can be used by the game to make Revolution nearly Inevitable.
So, shooting from the Lip, here's a possible sequence to introduce Revolution as the Starting Crisis of the Modern Age:
1. Greatly increase the Settlement cap for Exploration Age, to encourage wide-spread settlement as more of the map's land mass becomes available.
2. The very first Civic available in Modern Age is Enlightenment, and that triggers Unhappiness in settlements related to:
* Distance from the capital, with a bonus for being on a separate Continent/Distant Lands
* Age the settlement was founded - Exploration foundations being much more likely to Revolt
* Origin of the settlement - settlements not founded by you, as in conquered from other Civs or started as IPs: much more likely to go negative.
* development of the settlement: Cities with Quarters are less likely to grow upset than towns without any internal amenities
Nothing should be certain, but a settlement you conquered or converted from an IP in Exploration Age on the far side of the map from your Capital and in Distant Lands - you should probably plan to send a large garrison there and keep it there for the first part of the Modern Age (and a new form of Continuity should be
Place Garrisons allowing you to start the Age with those in place at fhe expense of your 'field' armies)
Another overlooked mechanic already in the game that could contribute to the mix: Migrants.
Settlements in revolt (Unhappy, burning down infrastructure, flying balloons as caricatures of The King, etc) could generate Migrants who simply don't want to live under the balloon anymore, and so leave your Civ's borders. Two such Migrants (a settlement in serious Revolt) could start a new settlement outside but near your borders, which takes the form of a Hostile IP.
-And a number of in-game events should reduce population in a settlement, such as Migrants leaving or Plague/Natural Disaster deaths - and that reduction should be reflected in Tiles Not Worked for lack of workers and therefore Food, Production, Culture, et al being reduced. IF Food is reduced below the current population, a mass migration/revolt may occur and you lose control of the whole settlement - and maybe, it starts a new Civ.
This population reduction and more specific Migrant action would also serve to make the Crisis Events in each Age far more serious: if losing population also meant losing tiles effectively and losing food and other 'bonuses' for the settlement, the point would be reached in many cases where a city becomes no longer viable and a Secondary Disaster occurs: it devolves to a Town or converts to another Civ or an IP and leaves your Civ entirely, Activities like these could also act as specific triggers for Civ change instead of leaving it to a Hidden Act of diabolical Game Magic: changing Civs should be a Gamer Decision, even when the Civ is doing so badly in the Crisis period that there really isn't much in the way of a viable decision left.
A "Revolution" mechanic for Exploration/Modern Age can be added without adding any new settlement types, just revamping mechanics already in the game but not exploited for revolutionary purposes. In addition, once Revolutionary Mechanics are in place, they can also be used to make all the Crisis periods more meaningful, to the point where they provide Decision Points for the gamer to either actively choose to keep battling on with his original Civ or change to a new one as a result of the crumbling of the settlements and structures of his old Civ.