Subverting expectations and suddenly aborting plotlines was a no-go zone for me for a while. The only time I could successfully abort a plotline was in a thought train.
After watching How To Train Your Dragon 3, I wondered what would have happened if the antagonist Grimmel was replaced midway. I came up with the outline of a story where he gets shanked by another antagonist, I codenamed Diavolo (yes, it's a JoJo reference).
Diavolo is a completely different problem. He saved night furies as a species (thought to be near-extinct in the movies, I pulled in the thread to actually have something new to work with) by carving his own path, using his own tools, not letting the law get in the way and if a few thousand people have to die, then so be it. A ruthless, cunning and pragmatic villain.
However, this wasn't an arc abortion, this was a great arc replacement. Compared to Grimmel, Diavolo is a much more interesting foil as he has a similar mindset to Hiccup (the protagonist of the movies), the only difference is that he casually marmelizes anyone who gets in his way and is more successful by doing so. As a good foil, he makes the protagonist question himself.
However, this was only possible thanks to Diavolo being forked from Anon (one of my OCs), who conveniently happened to be a good foil for Hiccup. Such a great substitute arc isn't always available.
I once created a standard D&D setting. In this setting there was an elf. This elf lived happily in the forest village until a green dragon came and killed everyone but the elf.
Following several misadventures the elf finally gets to take revenge on the green dragon.
Suddenly, the elf finds herself in a garden in her destroyed village, except it's perfectly fine, as if nothing has happened. Soon she spots the gardener, the green dragon, "Good morning [elf's name comes here]!" she (the dragon) smiles and waves at the elf with her wings, her eyes barely visible under the shade of the oversized straw hat she's wearing.
What happened? This is the power of [Lain Requiem], Demiurge's (another OC) invincible ability that grants him unlimited power over information, allowing him to create a new world by selectively imprinting and modifying already-existing information on the material plane. Don't rack your brain over it, it's complicated.
This aborts a main character arc forever, quite literally destroys the atmosphere and the world the reader spents some time familliarizing themselves with. That sounds similar to something.
Yes, the Long Night from GoT season 8, the one where Arya kills the NK. That moment nicely ticks all the points above. The reception can best be equated to an online lynching.
I wanted to do an arc abortion to set another plot in motion. Basically, the main characters are trying to stop the the dark lord. In one chapter they prepare for the final battle, in the next, they are woken up by their parents (who are supposed to be dead) because they have to go to school.
Turns out the chemistry teacher is the dark lord but is now humble and polite and has no recollection of the past world. Before @Amadeus bursts an artery, no, the previous world wasn't a dream, but it was wiped from the collective consciousness by Requiem.
The rest of that plot is dedicated to the characters trying to find out who turned their world upside down and try to turn it back. It's not that this storyline can't be interesting, but its tone and atmosphere does a 180° compared to the previous arc.
I don't want to drop every element of an arc (That takes away from the dramatic power of Requiem), of course, only its catharsis.
I'm trying to wrap my head around how can one effectively ruin an arc for dramatic effect (Pretty much what ASOIAF is doing) and to subvert expectations but not end up being hated by the reader for doing so.