system
Given the following post, respond as if you're commenting on Reddit:
human
[Dragonball] If Future Trunks dies in the regular timeline, which afterlife does he go to? The future timeline one or the normal one?
gpt
tl;dr:  the normal one, at first.

The Dragonball afterlife is an excessively complicated celestial bureaucracy, so keep in mind that things like 'justice' and 'common sense' may not apply.

Souls are physical, in a sense.  They don't have a recall function to send them back to their point of origin.  Individuals who die on a specific planet are under the jurisdiction of that planet's gods and go to that planet's afterlife (see:  Raditz, Frieza, and Dabura going to Earth's afterlife after dying on Earth, despite being alien visitors).  So Trunks' soul will remain in the timeline he dies in, and will go to King Enma (if he dies on Earth) or an alien judge (if he dies elsewhere) with jurisdiction over his place of death.

(Note that if filler is considered canon, we *know* that visitors from other timelines go to the main timeline's afterlife, because Cell, who originated in a possible future timeline, ends up there.)

(Also IMPORTANT EDIT:  If Super is canon, Future Trunks' original afterlife no longer exists to return to - the entire timeline was wiped out by Future Zeno and everyone in it ceased to exist.)

What happens to Trunks afterwards depends on how he's judged and how many friends he has in the afterlife.  Entities higher in the bureaucracy than mere planetary gods (King Kai, Shin, etc) can take jurisdiction and relocate souls to different afterlives (see:  all the Earthlings and Namekians King Kai takes to his planet), so one can expect that Trunks will end up wherever Goku, Vegeta, etc., are.  But changing timelines is much more difficult - as far as we know, only angels like Whis and Bulma's time machine are capable of doing it.  To get Trunks back home to his timeline, Goku would need to ask Whis for a favor.  (Whis reconstructed Trunks' entire multiverse after Zeno wiped it, so he'd probably be amenable to helping out one more time.)  If Whis wasn't around, option 2 would be to give Trunks a temporary body and let Bulma drop him off in his home timeline.  Either way, he'd probably make it home eventually, but it wouldn't be an easy or automatic process.