"Mainstream"

I don't recall anyone ever calling Shamala and Nightmare mainstream, even when I suggested moving them to their own page, I claimed they were sidequests and not to be confused with mainstream. Any I no longer think they deserve their own page because the justification of moving them to their own page was under the impression they were prerequisites to start the Saga.

I think Shamala/Nightmare, Puppeteer, and any other quest chain longer than 3 quests should be on it's own page regardless of weather we decide to call it mainstream or not, because throwing it on something like the sidequests page might make it look like the individual quests can be done out of order to someone who hasn't done it yet, and putting "finish the previous quest" in the "how to get the quest" box for each one of them would be messy and tedious.

Anyway, about your formal definition of mainstream, I'm not sure "first quest chain of the update" makes sense; Saga episodes 2-10 aren't the first quest chain of the update, unless you're counting updates smaller than a gen which would include Shamala, Nightmare and Puppeteer.

I was considering E1~10 the same quest chain.

But I agree, what we lable them doesn't matter. But if we give 1 "sidequest" its own page, they all deserve one, three quests or not.

My suggestion to give all quest chains their own page isn't about their importance, or weather or not they deserve it, it's about organization and clarity. I just think that if they're arranged one after the other ingame like a mainstream, then it would make sense to arrange them somewhat similarly to mainstreams on the Wiki. They'd just be in order on their own page with a note about how the first one is started, but if we don't decide it's a gen, then it still won't get things like the list of involved NPCs or any unnecessary links/references to mainstream stuff. For quests that aren't part of a chain, chronological order isn't as important so i still think it's fine to group them all on the same page with a note at the top saying they don't need to be done in any particular order(that part is optional since they have the "how to get the quest" box in the table anyway)

Even if there's a story order, I don't think we can call all of Saga a single quest chain if there's no requirement to do it in order.

That I agree on. So that begs the question, should Shamala and Nightmare still share a page? Ignoring whether or not they're mainstream.

Don't you have to complete Shamala to actually start Nightmare though?

Yes, but so do G1 to start G2.

They were released in the same gen, but different seasons, so I'm not sure how much sense it makes to have them on the same page since they came right after puppeteer, and even if Hwu somehow ends up meeting Shamala or Ruairi, g17s2 is still clearly a separate chunk of story from s3&4.