I suggested some things for him to do (all of them totally unconstructive and uncooperative) or rather,
not do in the Feanorian Storylines thread:
So, what if he just cannot, ever, forgive his brothers (other than Maedhros) for Amrod's death? And he just refuses to hang out with them, including Caranthir. Or even works at cross-purposes to them, probably not in sabotaging the Union of Maedhros or the attack on Doriath, but in lesser things. So instead of living just south of Estolad, he lives closer to Amon Ereb, far from his brothers. When they get together for hunting or councils or Mereth Aderthad, he doesn't come. When they send letters to him, his responses are cold, or nothing. When they want to trade with the Dwarves, he won't let the Dwarves traffic through his land on the way to Himlad or Aglond, prompting them to prefer the road that turns north before it reaches Sarn Athrad (which ends up helping Caranthir, but whatever). When they invite Men to ally with them, he pointedly doesn't. He isn't in any other way abrasive or unpleasant on a personal level. It's just that he holds a grudge against those particular people who happen to be his brothers. Like a personal, one-man feud.
Alternatively, we could have him start trading with Dwarves before Curufin and Caranthir do (perhaps on account of restraining himself from calling them ugly to their faces), only to have his brothers later poach the better trade agreements from him somehow.
Here's another idea: Morgoth started a lot of rumors about the misdeeds of the Noldor circling among the Sindar, leading to Thingol's confrontation with Finrod, Angrod, and Aegnor. What if a rumor about the burning of the ships was started instead by Amros? It would be a spiteful and negative thing for him to do, by comparison with Finrod's unwillingness to defend himself if it meant casting blame on his cousins... or would it simply be Amros' unwillingness to hide the ugly truth?
When Fingon rescued Maedhros and the Feanorians were finally convinced/dragged into reuniting with Fingolfin's host, I generally imagine that nobody wanted to talk about the burning at Losgar... or explain where Amrod was... except Amros who just blurted out the whole sordid tale. (Or Maedhros could explain, but I envision this scene happening while he's still too asleep/unconscious/out of it to converse meaningfully.)
I think a balance of regretting the Oath first of all the brothers, and being very antisocial towards his immediate family, could be interesting without pushing him too far either towards "total jerk" or "the most repentant of the brothers." Although since JRRT wrote nothing about his personality, "total jerk" wouldn't contradict anything, but I think it would get boring.
Also Erucheb's idea for Feanor's death scene is growing more on me.
He'll likely wind up saving one of his brother's lives in the Unnumbered Tears.
That could be an interesting turn of events. He wants absolutely nothing to do with this brothers, but when it's really important he clearly isn't so angry he wants them to die. Maybe shunning them, ultimately, is just making him more lonely. Then again, there's no reason to assume he has no friends.
He'll likely be the elf Finrod is visiting before his famous hunting trip where he discovered Men.
But this was Maedhros and Maglor.
Symbolism is only so valuable, but I think that Tolkien was saying things about the Oath and the Doom of Mandos in the way the Kinslayings played out. For one thing, none of the Sons of Fëanor is killed by an enemy (no orc, balrog, dragon, etc takes them out). In inventing the story where Fëanor burns Amrod in the ship, he preserved that, so this addition does not change that restriction. Thus, when we consider Amras' story, we should at least consider not allowing him to be killed by any of Morgoth's minions. Another very interesting detail is that those who instigate Kinslayings don't have long to live. Fëanor survives the first Kinslaying, it's true, but he dies in the next battle he is in, barely having set foot in Middle Earth before he is killed by balrogs. The 2nd Kinslaying seems to be instigated by Curufin and Celegorm. Dior's existence is particularly irksome to Celegorm (who wanted to marry Lúthien himself), and it is only when Dior holds the silmaril that they attack Doriath. Who dies in Doriath? Celegorm, Curufin, and Caranthir. The other brothers participated, but it wasn't (presumably) their idea, and they lived to go on. At the Havens, Tolkien has the twins as instigator, and Amrod and Amras dying there. For the 4th and final kinslaying, we actually have dialogue for Maglor and Maedhros' debate ahead of time, which makes it clear that Maglor is reluctant and Maedhros is pushing for it. They survive the kinslaying, but Maedhros dies after he touches the silmaril (while Maglor, at last, repudiates the Oath by throwing the silmaril away).
This is pretty close to my interpretation of things, too. The brothers who survive each Kinslaying are those who are closest to repentance (or remain the most repentant.) I think they were being given chances to try to break the Oath. (Although this stops looking merciful once Maglor is wandering alone in eternal pain, but he could kill himself at any time and in some versions he did so.)
Amrod did try to break the Oath... and maybe death
was a grim mercy to stop him from actually incurring the full consequences of that. He certainly didn't end up tangled in any more Kinslayings...