I’m writing Mycroft as aromantic asexual here, which is a slightly prickly topic to tackle in a soulmate AU.
I love these AUs, but it's my first time writing one, and I’m still working out my notions of how they work- but I do know that I think that in a universe where people have a soulmate who they have
to be with, whose name is written on their skin, most of the prejudices- or just general lack of understanding- that people who are not strictly heterosexual face wouldn’t exist, because, as a concept, sexualities wouldn’t really exist.
Two men might love each other, or two women, or a man and a woman- and their relationships might be sexual, or romantic but not sexual, or strictly platonic, depending on what the two people in question happened to be suited to.
They are soulmates, after all.
(I’m still not sure how to deal with poly in such a universe, and under the circumstances I’m not touching it.)
Besides that hiccup, though, it’s easy to romanticize such a universe, and since I was playing with the question of under what circumstances might Q feel comfortable- even happy- with the notion of ignoring his soulmate, I delved into what kind of weaknesses such a society would have.
And since I couldn’t deny the possibility of people who didn’t have a soulmate, or had what we might call an “unrequited” soulmate, I thought that these would be the groups that society at large was only just beginning to accept and understand.
Which brings us to Mycroft, whose... let's use the word "platonic" though I'm not quite sure it's the one I want, soulmate is his brother. I originally wanted to give this story some kind of “unrequited familial love” tag, given that Sherlock’s soulmate here is John- but the fact is that, as in canon, Mycroft’s love for Sherlock isn’t unrequited, it’s just that Mycroft is single-minded in his not entirely healthy love for his brother, and Sherlock is not. Sherlock has his cases, and he has John. Mycroft has other things in his life too, but nothing else that he truly seems to love.
At any rate, in what was meant to be a short note made entirely too long, what I ultimately want to do is cover myself by saying that Mycroft’s asexuality is more an 'in addition to' his being a person without a “usual” soulmate, rather than a 'because of'. I do not think that people who are asexual- even when they are also aromantic- would of a necessity have no soulmate, or have to pine- platonically or not- after someone whose soulmate was someone else. That said, I do think it might happen sometimes, and it's what happened here.