Working with gods again, trying to whip together something resembling characters since they're going to be important to the plot soon, so question; just how tied to their themes are Terrestrial Gods and how powerful can they grow?
I've been working with gods for some of my own work, actually. I'm not sure what you mean by them being "tied to their themes"; to me that could mean either "can gods have opinions and concerns and personality traits beyond their assigned domain?" or "are gods' powers completely defined by their assigned building/lake/etc, or can they have Charms that work with concepts outside it?", so I'll try to answer both.
Personality-wise, the party line is that gods tend to be more human in how they think and act than demons of similar power (so the god of Nexus is going to have less weird quirks and alien value judgments than an SCD, and a minor dream-god will be more "normal" than a First Circle demon that does things with dreams), but they do tend to have strong Intimacies connected to their domain, and their Motivation is usually about expanding or protecting their assigned domain.
I generally handle that by making them come across as being a person who has a four or five-dot Intimacy for their domain, so that whatever priorities or Virtues normally define their behavior get thrown right out the window if that domain-Intimacy triggers
[1]. Beyond that, what they value and how they conduct themselves is entirely up to the individual - at most, you'll see a trend toward shared interests or personality traits among gods that frequently associate or come from similar backgrounds.
Speaking of which, it's important to remember that a given god hasn't necessarily been assigned to their current domain all their lives. A city-god might have started out administering the cliffside their city was later built on, and either absorbed the duty of managing the city into their previous workload or abdicated their position as cliff-god to take up what they saw as a more attractive position - or vice versa, with a city-god outliving its city and managing the land where its city once stood, so that it goes from a god of ruins to a god of ruins being eaten by jungle to a god of jungles as the city's remains are worn away over time. Deities forced out of their original position by Yu Shan politics might end up reassigned to
Siberia one of the Directions, or decide they'd rather lord it over a tract of Creation than retire in disgrace to Heaven's slums.
The tumult of the previous Age provides any number of ways for a god in the Age of Sorrows to end up going through multiple careers, and that will leave its mark in their Intimacies. The forest she once ruled might have been swallowed up by the Wyld, but that morose lackey to the local city-god will still enjoy being around greenery, and a god of brewers who ends up administrating a mountain pass out in the boonies will look kindly on those who offer jugs of ale or hard liquor at his shrine.
Which leads into the second interpretation - yes, Terrestrial gods can absolutely have powers outside their official domain. For example, that former forest-goddess could have Charms that involve manipulating the native flora and fauna of the city she now helps oversee, as well as Excellencies for Survival, Medicine, Archery, or other woodland skills, and the brewer-cum-mountain god might still be able to punch you in the brain with a magically-induced hangover.
(
@EarthScorpion adds to this with the idea of gods often having special "regalia" items that symbolize their position & provide increased power for the god it belongs to, so you end up with the divinity who oversees a militaristic tribe hauling a wicker cornucopia around from her previous job as a harvest god and using it to magic up food for her worshipers, or a disgraced god clinging to the soiled robes of his former office - and the power they provide him, even in their diminished state - as the only remaining piece of the wealth and influence he once wielded.)
Another way Terrestrial gods can have Charms that don't directly sync to their domains is by invoking traits or associations specific to their territory. The god of a plot of badlands frequented by bandits might eventually absorb themes of victimization, fear, or violence and manifest them in his Charmset; a hill-god could use Regalia of Authority by "borrowing" the intimidating presence of the Shogunate ruins that loom over a nearby town from atop its summit; the actions of a murderous cult could gradually let the murder god who adjudicates their string of killings start cribbing Charm ideas related to whatever the hell this cult is killing people for. This both further distinguishes gods with identical general domains from one another
and offers encouragement for gods to encourage or suppress certain behaviors among the mortals they interact with. That badlands god might well take poorly to lawmen trying to clear out the bandits who hide on "his" land, for example.
[1] So you can have a plains god who's incredibly easygoing & appreciates fine clothing (because he spent time as the aide to the god of a city which once stood where the plain is now, and still has a certain taste for the finer things), but swiftly gets Biblical if he finds people squatting on 'his' land without providing offerings in exchange, or if a city's fashion industry results in byproducts from dyemaking poisoning the groundwater on one edge of his territory - because if the plains he manages are impinged upon by others, his Compassion 2 & Intimacies for fine clothing and not causing a fuss get shoved out of the way by his 5-dot "these plains are MINE" Intimacy.
I mean, I have a super-potent Dog Of The Unbroken Earth called Furrow-Filler who turns any dog that comes into the region (that extends anywhere from the size of a modern day urban sprawl to the entirety of Ireland from day to day, I really need to get a grip on the distances involved) into a savage killing machine that slays its master and flees to the bog marshes. His cult shows devotion by fighting these hounds with their bare hands and consuming their flesh. They even have a potion derived from the tainted brains of these beasts that drives them into a killing frenzy.
I just wanted to single this out and say it's a damn cool idea.