Story Update:
So side quest will also be important for the main story (kind of). Basically every character, even ones you think may not have much in common with the main story, kind of do.
Ok, they don't have meaningful impact, but they know the main characters in the story and might have met them. This is where the ACT system and keywords come in to play, because you can talk to these side quest characters about their experience with the main characters, or at least what they know about them. This is also a great way to get info on some characters that you may want to figure out. Which is very unique compared to most NPC's you talk to in other RPG's where it feels like they have no input on the world around them.
It's definitely a challenge to make a story for an open world game, but I think I did a pretty good job of doing it. The ACT system and keyword system really help for people to get info on the story without making it linear.
I wished I talked about this sooner as it was something I've already been implementing.