Since we only see Ornn/Voli/Anivia hanging out in the Freljord, does that mean they only have domain in this region? Did they help shape the whole world, or just the Freljord, and if the latter, will we learn about other regional demigods and their influences?
I think the Runes shaped the world, after all the planet is called Runeterra. The myths about 
are just that, myths that people passed down through generations to explain things they don't understand about the natural world. I like that Riot actually designates the "demi-gods" as greater spirits, basically spirits so powerful that when they manifested people mistook them for dieties.
That's a major theme in LoL. The Ascended were mislabeled as gods, The Aspects are revered in much of the same way because people are ignorant of Space Targon on Mount Targon and even Aurelion Sol is called a god because he inadvertently created life by creating the stars, he had no plans for us it was just happenstance.
I think they only shaped the Freljord, but I think they have power elsewhere as well.
I mean, I'm assuming Anivia can just summon ice out of nowhere, Ornn can summon fire out of nowhere, and Volibear can summon lightning out of nowhere, otherwise they are extremely anticlimactic for what is supposed to be a god-like entity.
Other (potential) regional demigods could be someone like Rammus, who could've shaped the deserts of Shurima with his tremors.
I think it's Runeterra-wide, but they typically hang in the Freljord. I'd imagine there are demigods for different regions, but maybe there is an explanation they aren't around.
