I don't know the entire up-to-date-story of Genshin Impact, as I just finished the storyline of Liyue yesterday. Due to that, I won't be mentioning anything past that point in the story, other than the few spoilery theories that I've heard of.
In this, I will refer to Lumine as the missing twin, and Aether as the main character. I only loosely understand it, as I wanted to avoid spoilers, but I want these reactions to be based off of the idea that Lumine arrived to Teyvat years before Aether did. (I'm not sure if this is fact or theory.) Lumine will react to Aether's various adventures, and if you want you can come up with a story for Lumine, based off of what we know so far, that Aether can react to. Have the playable characters, the archons, and anyone relevant to the story react to Aether's adventures as well, and have the cast try to theorize about the unknown god that the twins encountered, the general lore of Genshin Impact, and other characters' backstories; put a lot of focus on all of that. Have the characters' reactions to the twins being from a different world/worlds be interesting, and also have them theorize on just who and what exactly Paimon really is, whether it be her being an archon of time with lost memories or some other theory that's out there.