So I heard that the Utawarerumono PS2 official guidebook (2006) had an interview with interesting info in it, and so got it. Just translating/summarising some snippets.
NOTE: I CAN'T AINU SO I'M PROBABLY TRANSLITERATING A LOT OF NAMES WRONG.
Washimi Tsutomu: Director
Suga Munemitsu: Writer
Amaduyu Tatsuki: Character designer
Utawarerumono takes place in Japan (in the future). The Earth's axis has changed a bit in this world. The rest of the world of course still exists, and there are other races not seen in the game living on other continents. -Suga
There were initially plans to have Benawy, Crow, and/or Oboro die at the end. Crow even had a scene written, where he falls off a cliff along with an Av-Kamu yelling "long live lord Hakuoro!". -Suga
Q: What is Hakuoro, anyway?
Suga: To put it simply, "someone with power" (力のある者). He is called a god, but this isn't the sort of god found in contemporary religion, but rather something with a physical form and overwhelming power. Hakuoro's body is that of the archaeologist which by fusing with the "something with power" (力のある何か) became a god. As for what this "something with power" itself is... Well, I do have an idea, myself, but I'd rather leave it up for the players to decide.
The white and black gods fought a war back when Tuskul was a young girl too, and she was on the side of the white god. The white god called himself Hakuoro back then. Eventually he lost and was sealed away, but when Tuskul grew up and gave birth to her son, she named him Hakuoro after the white god. When Eruruu brought Hakuoro home, she immediately realised that he was the white god, and gave him back his real name. -Suga
The tails are an extension of their bums (referring to the scene where Eruruu says it's rude to touch people's tails) -Amaduyu
All of the women in Yamayura have a ring modelled after Mikoto's master key, which they are free to wear wherever they like (Eruruu uses her's as a hair ornament). Eruruu's is the original, which was passed down through her family, as direct descendents of Mikoto. -Suga
Aruruu's power to speak to animals originally comes from Mikoto. It's passed down genetically, but not everyone in the lineage can use it. There were initially plans to have some stages set in Iceman's age, and Mikoto would have used this power there. -Suga
Yuzuha didn't die from her sickness, but from the strain of giving birth. -Suga
The magic used by the onkamiyaryu is actually Mutsumi's psychic power, passed down to her descendants. -Suga
Oboro is actually royalty, the grandson of the previous lord of Kenashikourupe. Kenashikourupe fought on the side of the white god in the previous war, and because it lost the royal family lost its position as well (to Inkara's father). Dory and Grah are descendants of a clan that traditionally serves the royalty. Benawy is related to the royal family and thus Oboro, and when Oboro named himself he immediately recognised him as the true successor to the throne, and this is why he let him go. -Suga
Genjimaru's contract with the black god was solely for power, which is what maade him this strong. -Suga
The Shakcopol have physical strength equal to that of regular humans. -Suga
Only the Shakcopol can use the Av-Kamu. -Washimi
The Av-Kamu were originally robots that the humans made for the Shakcopol to use for manual labour on the surface. They don't have any built-in weapons because they were originally industrial machines. -Amaduyu
Onkamiyamukai's lord Wabe also fought on the side of the white god in the previous war, and knew Tuskul personally, which is why he was surprised when he heard the name of the new country Hakuoro founded. -Suga
Chikinaro is actually working for Dee, and it's unfortunate that we were unable to expand on this in the game. -Suga
Because the white god is the one with the archaeologist's body, the black god is a disembodied consciousness and has to get his own separate one. Dee couldn't control his curiosity and wanted to see what the god the Onkamiyaryu worshipped was like, and entered the chamber where Witsalnemitea was sealed, and awakened the black god, who said it would grant a wish. Dee asked for knowledge, and the price was his body. -Suga
Nuwangi was originally supposed to be a rival character to Hakuoro, like Char to Amuro. -Washimi
He was originally going to stand out more, being a pervert who would chase Eruruu around in his underwear, but other people on the project stopped me from writing him this way, and when I had him calm down a bit he was a pretty boring character, and I ended up writing him out earlier. -Suga
That resulted in the creation of Dee. -Washimi (tl;dr the guy voiced by Char was the replacement for a guy supposed to be Char)
The previous war ended in the black god's victory, thanks to Genjimaru and Karura's father who fought for him. Originally the white god, with Wabe, Tuskul, and Oboro's grandfather on his side had the advantage, but the tables were turned after "something" happened. -Suga
Amaterasu/the light of purification is a weapons satellite. There were multiple Amaterasu launched as part of a project to make an artificial sun. The demi-humans had already been living on the surface for some time when Mutsumi used the satellites to try to destroy Witsalnemitea. Also, the Shakcopol had still been living underground with the humans up till this point, and it was only after the underground facilities were wrecked that they came up to the surface, where they were subjected to discrimination due to previously having had special treatment from the humans. -Suga
There may be some surviving humans left who hadn't been turned to jelly. -Suga
We're thinking of making a sequel. -Washimi
I already have a general plot in my head. The protagonist isn't Hakuoro, though. -Suga