What are some concepts, creatures, places, cultures, items, whatever from other media that you've considered translating into Forgotten Realms? It doesn't have to be something you've seriously considered and it doesn't have to be that serious. Would the Incredible Hulk work better as a Barbarian/Green Star Adept or some sort of Giantkin Bloodrager? Who knows?
Eldar Spirit Stones
I like my WH40k lore and my dumb weeaboo space elves. That's no secret. I've thought about multiple ways some sort of Eldar-ish culture could exist in FR, from some kind of Star Elf variant, to bringing in the Imperial Elven Navy of Spelljammer. For the moment I've decided that the Gith--specifically the Githzerai--are close enough to some sort of stoic, vaguely elven-looking(they're actually human-descended, yes, I know), alien and space-/plane-faring race for my tastes. They have a troubled past, they have multiple "paths" that they follow in life and often blend them(Zerths), and they rarely follow gods(Eldar have Khaine and that imprisoned goddess I guess... not sure how I feel about the Ynnead stuff yet).
Anyway, they share a few similarities with the culture I think is pretty neat in WH40k. One of the items I particularly like from there is the Eldar Spirit Stone, a gem the Eldar wear to capture their souls at the moment of death and prevent them from being devoured by Slaanesh. Later, they add these stones to their giant city-ships, where they become part of a matrix; this is the closest thing the Eldar get to an afterlife.
While the Githzerai don't really face such a danger to my knowledge, it makes sense that they might also have a way to capture their souls. They have no great patrons to look after their souls when death comes; plenty of demons and devils would jump at the chance for unguarded souls. I don't see a soul floating around in the chaos of Limbo surviving on its own very well. If Vlaakith, the lich queen of the Githyanki, is any rubric to go by, their souls can indeed be consumed, if not by some god or demon, then by some sufficiently determined undead. I really don't think Kelemvor or any of the Prime Death gods would have a dominion over them, but in case they did, being Faithless would mean they'd be added to the Wall upon death. With all those possible post-life threats, it stands to reason that they might have developed a method to combat this. Moreover, it would likely be arcanely crafted, seeing as Gith clerics aren't really a thing(despite what NWN 2's campaign will tell you). This is how I see it working:
Eldar Gith Spirit Stone
The What: Stone that absorbs a soul upon the holder's death, preventing spectral fuckery
The How: Craft Contingent Spell + Soul Bind + Black Sapphire Gem
There you are. A gem that traps souls on death, sparing the Gith from being destroyed/eaten/lost. I'm not sure if that gives them an actual afterlife, though. They'd be trapped in their own gem. They of course don't have craftworlds like the Eldar to go back to. I could see some sort of big post-rrakma ritual where Githzerai Wish all the Blak Sapphires they've collected into a larger gem, perhaps a huge gem or artifact that serves much the same purpose of the matrix. Then you have all the souls together, sharing eternity with their battle brothers/sisters, family, and ancestors. Still trapped, I guess... but better than being obliterated by Limbo or a dozen other hazards.
), that was heavily inspired by Legion Commander in Dota 2. It was a Paladin from Krynn, following a god of war and commanding as she, the paladin was wielding an halberd. She would hit like a train in her full plate but was pretty horrible if someone focused her. Over zealous, Always ranting at others poor organisation and looking for ways how to improve how to fight.