In my opinion, they're both great in their own ways. The story of Xenoblade Chronicles 1 was classic and engaging, while Xenoblade Chronicles 2 has its own unique charm and depth.
Since there isn't a lot of official information about Xenoblade X Online Story, it's hard to say for sure. But in the original Xenoblade X, we had characters like Elma, who was a key figure in the colonists' survival efforts. If the online story is an extension, she might still be a major character. There could also be new characters created specifically for the online experience, perhaps characters that players can customize and develop as their own avatars in the game.
Irys is also a great character for fanfiction. Her role as a Nopon merchant gives her a unique perspective. In fanfiction, she could be involved in some sort of trade deal that has far - reaching consequences for the colony on Mira. Or her knowledge of the local lore and secrets could be the key to solving a major problem in the story.
One common theme is friendship. Just like in the game, fanfics often explore the deep bonds between the characters. For example, the relationship between Rex and his Blades is further developed in many fanfics.
The story length of Xenoblade Chronicles can vary depending on how much side content you explore. On average, it takes around 50 to 80 hours to complete the main story.