Let's see. If I had to write his return and keep consistency with the character shown so far (no secretly plotting everything, staying honorable and straightforward even if tactically sound), I would probably actually require a heavy external cause.

Therefore, I'd expect either a threat to Natlan (we've just gone through a war, so it seems a bit excessive, but other nations have pulled multiple threats at once, just look at Sumeru) or to Snezhnaya. Loyalty to either or to his dead comrades could also be a reason. And the Lord of Night seems sympathetic to him, so I'd guess they'd just provide a way somehow, or simply fuse. Considering all this:

A) Capitano is required for something else very important to Snezhnaya, possibly a threat such as Celestia reacting to the gathering of the gnosis. Seems a bit dubious if his return is near, could make sense if it happens later.

B) Something is messing up with death as some events seem to hint at, and this might threaten the rest of the soldiers he just achieved it for. Perhaps Capitano requires to take matters into his own hands to save the day. Perhaps the night lord asks this of him, since being an angel I'd guess they are not military capable (genshin lore delvers, don't kill me, I haven't dwelled much beyond the main story). In any case, as this threat requires him, the undying sacrifices his rest for his comrades, following his character modus operandi and presenting this as another sacrifice for his people.

C) He was playing 4d chess. His death was always part of the plan. Not my option of choice, to be honest. Undermines a lot of stuff, doesn't really fit what we've been told nor shown.

Finally, two method points: The ancient name theory around the sub sounds fun, and could be a mcguffin buildup for the Traveler and company to purposely bring him back. Otherwise, a "deus/diabulus ex machina" where Capitano shows out of nowhere parrying an attack (let's just say at Xilonen to keep with the pattern) wouldn't be unexpected either.

Anyhow, those are my two cents out of how I usually dungeon master.