Rather than straight up "monsters" to fight and kill indiscriminently, I suspect we need to go more in the direction of "enemy combatants". As Theros is based off of Ancient Greece... the theme of warfare between city-states stands tall. So I think fighting against other poleis and their military should probably have a much more prominent position in terms of the combat pillar. Which means fighting other humans, centaur, minotaur, tritons, satyr, harpies, Returned, and medusae would all be your typical opposition should combat arise.
And why would combat arise? Because of the duties PCs have towards their gods and their poleis. Combat will more often than not probably be political action, not for treasure-seeking. Theros has "heroes", not "adventurers"... they shouldn't just be going out into the wilderness to hunt monsters, raid tombs, and look for gold. That's the format of your standard D&D campaigns, not the standards by which I think Theros was meant to be played. Therosian heroes should be fighting for causes, fighting for their piety and their gods, or battling for reknown. Thus needing "low-level monsters" shouldn't actually be necessary-- Therosian heroes aren't going out to raid goblin camps, they are going out with their phalanxes to defend their walls from the triton warshoal trying to enter the polis to acquire weapons and the like.
And why would combat arise? Because of the duties PCs have towards their gods and their poleis. Combat will more often than not probably be political action, not for treasure-seeking. Theros has "heroes", not "adventurers"... they shouldn't just be going out into the wilderness to hunt monsters, raid tombs, and look for gold. That's the format of your standard D&D campaigns, not the standards by which I think Theros was meant to be played. Therosian heroes should be fighting for causes, fighting for their piety and their gods, or battling for reknown. Thus needing "low-level monsters" shouldn't actually be necessary-- Therosian heroes aren't going out to raid goblin camps, they are going out with their phalanxes to defend their walls from the triton warshoal trying to enter the polis to acquire weapons and the like.