OMG sorry @ladytyburn XDDD but okay, thank you so much for asking about my AU! <3 I decided to make this a separate post rather than drop lore in your notes because apparently I had a lot to say once I got started?
(Soulmark AU infodump below the cut!)
Okay, so, Soulmark AU starts from the basic premise that for chakra users specifically (and only chakra users), fate matches you with a soulmate. Every shinobi, samurai, etc is either born with a soulmark or expects to develop one as they grow up, depending on whether they or their soulmate happen to be born first. The mark consists of one or more black kanji, like a tattoo, on the inside of the wrist of the dominant hand; the specific kanji you have will relate to some fundamental quality or trait of your soulmate, but their symbolism is often super cryptic and obscure until you actually meet the other person and go "oh, I get it!"
Speaking of which, when you meet each other, you'll know, and the thing you're meant to do at that point is to grip each other's forearms so your wrist marks touch. On doing this, you will both experience a neurochemical response that biologically calibrates you to your soulmate and dumps a ton of bonding hormones and so on into your bloodstream. Newly bonded soulmates will, if left to their own devices, collapse into a heap of fluff for several days and then emerge shiny-eyed and ready to take on the world together. Culturally, soulmates are recognised as Very Meaningful and bonds are generally respected, but there can definitely be cases where an awkward or inappropriate bond causes drama, outrage, or political disasters (which is where the fun starts, of course).
Random facts of possible interest:
Soulmate bonds often develop into sexual connections just because of the sheer biochemical intensity involved, but it's completely possible to be celibate or platonic soulmates (because of the whole "fated compatible match" thing, if one partner isn't interested in sex for some reason, the other generally won't be either.) Sometimes siblings can even be soulmates, which due to them being raised together tends to naturally create a very different dynamic to the one you get between soulmates who meet as adults. As a rule, the younger you are when you meet your soulmate, the more smoothly the bonding process will go (especially if you haven't gone through puberty yet) and the less emotional upheaval will accompany it.
Soulmarks are connected to some unique anatomical features: there's a small gland in the wrist directly under the mark, which secretes bonding hormones, plus a few other associated glands and things and the bit of the brain that controls them. This means that while soulmate bonds are both spiritual and physical, it's possible to destroy the physical side of a bond by surgically removing someone's mark and glands. This is severely culturally frowned upon, but is practised by some black-ops type organisations, including Root. If it's done before you meet your soulmate you will never be able to bond with them normally in this lifetime even if you still recognise each other, making this genuinely one of the worst things you can possibly do to someone. Losing your mark to accident or injury will, of course, also impact your ability to bond with your soulmate...
The soulmark appears on the dominant hand, so in order for the matching wrist grip thing to work, it is Just How Things Are that your soulmate will always have the same dominant hand as you. This has the side effect that if you're naturally true fifty-fifty ambidextrous, you may have two soulmates, which is very rare but not unheard of. Some people in that situation just get another ambidextrous person as their soulmate though.
If your soulmate dies, your soulmark will fade to a dull red. If you were already bonded to them you will suffer extreme grief and physical withdrawal symptoms; these are survivable, but not everyone chooses to live on without their soulmate. If you hadn't met them yet you will experience only an echo of the physical loss, but intense grief is nonetheless a normal and accepted reaction.
It's pretty universally believed that soulmates are a For All Time thing, so following a soulmate bereavement, finding each other again in your next lives is commonly hoped for and expected. However, sometimes, a red soulmark may return to black, meaning your soulmate has either been reincarnated early, or physically returned from death in some fashion. This is mostly just a popular romantic fiction trope, but does sometimes happen in real life too, especially if you lost your soulmate very young (or live an unusually long life).
As for what all this has to do with Naruto and Gaara, to come back to the original point; I don't want to spoiler too much because they're meant to be the next chapter in my ficlet collection, but yes, they are soulmates, mostly because I love how loyal they are to each other in canon and how much they understand each other in ways nobody else does or really even can.
Random fact: Gaara's forehead kanji is actually a copy of his soulmark in this AU! He and Naruto meet at the start of the Chūnin Exams as per canon but things go, uh, very differently, which is obviously going to have a knock-on effect across the plot of the series in general though I haven't worked all that out yet. Because they're both jinchūriki, their bond is going to get complicated for sure (please picture Gaara and Naruto hugging each other ferociously while Shukaku and Kurama glare at each other and facepaw in the bijū-dimension) but I fully intend for them to get a lifelong happy ending, and your comic just made me think of how they'd be with each other in that AU so I tagged it for personal inspiration. Thank you for the lovely art and indeed thank you also for chasing me about this!
BTW if you want to read what I've actually written for this AU, the ficlet collection is here. Pairs I've written so far are Hidan/Jashin and Hidan/Deidara (the ambidexterity rule applies for Hidan!), Sasori/Third Kazekage (my favourite tragic OTP for Narutoverse) and Kakuzu/Shikamaru, which is my best friend's fault and the direct result of a fic they haven't posted yet because I'm still betaing it >_>. I'm still working out what a lot of the soulmate pairings in this universe ought to be though, so this fic will update very irregularly.
Thank you again for taking an interest and if you want to ask me more about any of this, please do, I love to ramble about my AUs!
