A computer game, either TBS or RTS.

Setting: fantasy, magic is abundant, the world looks like Earth but politically, economically, and physically/biologically, it is in fact not at all like Earth.

The player has a team of a few (I feel 4 is the sweet spot) "Questants", members of an advanced and expansive society, well-trained in magic, technology, tactics, combat, diplomacy, etc. Basically, max-level adventurers with government support, high education, and great preparation. Each has a specific set of traits to use, skills to teach (!), spells, "craft recipes", and "blueprints".
Teams of Questants are sent to lands with useful resources and primitive natives. The mission of Questants is to claim the territory, and it's dwellers (if any) for their empire, using any methods they need or want to use. Peaceful annexation of the willing, offering help with problems, political marriage, buyout, bribing the leaders, enslavement, magical brainwashing, are all acceptable for expansion. If everything fails, banishment from territory or extermination.

The game maps have a large amount of villages on it. They are not just "three buildings and two scripts", they have (if rudimentary) internal economics and politics, and a set of relationships with other villages, problems, local specifics, and so on.
A team of Questants under a player's control appears on the map with a mission to claim it. Or, with a mission to get rid of enemy Questants.
Questants can go into villages, and interact with leaders and population and whatever. The motive is to get them to join you, and your dream adventurers from an advanced empire can very well conquer villages on their own, but that often means than you get a crippled village with half of working population gone, the other half rebellious and demoralized, and the rest of the villages immediately wanting you gone, with a possibility of uniting together against you, or, even worse, welcoming the other Questants as defenders and liberators. Sometimes you need to "complete a quest" from the leader, but sometimes, it will be unreasonably hard. Sometimes, you can win the harts of villagers by waging war against their rival village, which is a "2 for 1", but that also may give you an unreasonably bad reputation. Etc etc etc.
Once the village is yours, you can repurpose it to be... basically, your base. Questants may teach villagers some of their martial and magical skills, set up the production of their battlegear, rebuild village halls as castles, etc etc. Villagers are your "normal" strategy units, and Questants... basically determine the tech tree and catalyze unit "production". (Villages don't magically produce a new guy every five seconds, villagers are very much a deleptable resource.)
And of course, once you have your first "base", you can recruit immense militia to conquer other villages. Or an economy to bribe them to submission. Or train an army of revolutionary leaders and send a few to every village they can reach. Or harvest magical energy to cast some dumb global spell. The recruited villagers are (in most cases) controlled indirectly, through actions and orders of Questants.

Basically, it all solves a few old problems with strategy games (while, of course, creating about as much new ones):

Strategy games often have bases build by a few guys in a few minutes/days. Here, all buildings are pre-built, and can only be modified and repurposed.
Strategy games often have units made on the battlefield directly out of resources. Here, troops are recruited from natives, and quick training is justified, because natives are very primitive compared to Questants.
Strategy games (esp. RTS) are often straightforward with gameplay - build an economy, amass an army, attack ftw. Here, you have to play a game with personal interaction, diplomacy, and adaptation to "local political landscape", before you get a chance to raise an army. OR you can try to single out all enemy Questants - that works too.
Startegy games with factions usually have them set in stone, and strategy games with extensive customization (i.e. deckbuilding or skill/inventory building) usually have a few dominant, finely-tuned strategies. Using a combination of a few characters as a "tech tree" should be somewhat of a middle ground.

I've wanted to see an RTS with pre-worked settlements and economics (why do war games start with building an economy from scratch, that's not how war works and RTS are mostly about war).
I also like Questants as "tech trees" especially if you mean that literally. Having a questant in a settlement gives it new research options. And I suppose once settlements learn it well enough they gain the ability permanently and don't need the questant any more. Maybe they can even send scholars to other cities to teach them too.
And I suppose Questants can't teach just any skill to any body. They need to work with what they have. Different settlements/cultures/regions are more predisposed to develop certain tech.

Looking a this again it reminds me of the Safehold series- scifi novels about an android trying to save colonists on a far off world that live under a tech-repressing theocracy. It also reminds me of some other book series I only ever heard about where warring societies from the future both send back representatives to mold the past and influence their war. It was Byzantines with aid from the protagonist vs ancient India with backing from future fascists if that helps anybody pin it down.
I think it would be a good general direction to take the flavour for a game like this.
For an old world feel give it some kind Prometheus myth narrative. Multiplayer would be gods competing to guide/lead/conquer humanity.
A 3rd option would be post apocalyptic.