I have an idea for a novel. It follows a stupid gimmick, would be between 10,000 - 20...

I have an idea for a novel. It follows a stupid gimmick, would be between 10,000 - 20,000 pages (longer than any novel currently published) and it will take me between five and fifteen years to write. And I won't tell you what the gimmick is.

But, since the novelty doesn't demand any particular story (aside from its length) I'm looking for suggestions. What classic story format can be used to write an obscenely long novel that's written blindly across what will likely be a decade?

I think the "high fantasy" genre makes the most sense. Many pages can be dedicated to worldbuilding, and establishing many story lines. But the work is a linguistic project that demands modern word usage in some cases, so it would have to parallel our world to some extent. Perhaps a hybrid of science fiction and high fantasy, then.

Just write a multigenerational epic in extreme detail following 250 years of an American family

Easy to combine the two with a society which has developed science out of its magic.

Good luck

boring. trite. vapid.
the superior choice is obviously a 20,000 page detailed history of a modern civilization that became advanced, regressed (for any reason OTHER than war), and believes the remnants of that advanced civilization is magic.

bad luck

That'll take you way longer than a decade to write, OP. More like a lifetime plus.

>OTHER than war
anime and porn

>>OTHER than war
> anime and porn
uh, no. War is the go to reason for that plot, YOU FUCKING CHILD. War is a cop-out reason and everyone knows it.

Oh~ there was a war~ because... because... HUMANS are... BaaAAAad and RruUUuin everything~ OOooh~

All three of those things seem to be exactly what OP is after.
The nature of magic means the society could only legitimately be one thing: they are all imprisoned in the matrix by whatever means.
You know what could work though. If it is only a limited number of some devices.

For example, a solar powered laser pointer. It turns out to mark its targets and a death ray satellite will kill any marked targets.