jiang cheng has been in love with wei wuxian for as long as he can remember.
however, as far as he can tell, wei wuxian is straight. when a troupe of actors puts on a play that includes a crossdressing side character, wei wuxian finds the character hilarious and cracks jokes about the character for weeks; during the cloud recesses lectures, when nie huaisang shows them a cutsleeve yellow book as a joke, wei wuxian laughs so hard he nearly throws up and immediately plans on showing the book to lan wangji - specifically to disgust the latter. jiang cheng resolves to keep his feelings to himself.
events proceed as they do in canon. wei wuxian dies.
events continue to proceed as they do in canon. wei wuxian comes back to life! wei wuxian tells jiang cheng to leave it all in the past.
postcanon, wei wuxian and lan wangji get publicly engaged.
life goes on. jiang cheng continues to dedicate his days to the running of lotus pier and the support of his nephew. with jin ling busy in jinlintai and unable to visit as he needs to appear independent, his life is lonelier, but he has all his work as sect leader jiang to keep himself occupied. people still gossip about sandu shengshou's eternal bachelorhood, but - as he has always done - jiang cheng lets them talk. nothing they say is something he hasn't heard before, anyways.
one day, on his way back from a solo nighthunt, jiang cheng - tired of humanity - decides to camp for the night in a cave.
as it happens, the cave is occupied by an incubus. the incubus is currently rather weak and so had not planned on eating anyone tonight, and was instead only sheltering in the cave. however, the incubus - being rather confident in its own abilities - sees no reason to back down from an opportunity to feed, even if the prey is much stronger than usual.
thus, the incubus transforms itself into the visage of jiang cheng's love - wei wuxian.
naturally, jiang cheng immediately IDs the incubus as an incubus, and a rather weak one to boot. he figures that exorcising it should not be a problem at all. but when the incubus - transformed to look like wei wuxian, who jiang cheng knows is now living for good in the cloud recesses - makes its advances towards jiang cheng, jiang cheng in a moment of weakness relents and allows the incubus to do as it pleases.
oh well, jiang cheng thinks afterwards, he can always exorcise the incubus later.
the next day, jiang cheng leaves the cave. he does not exorcise the incubus.
wei wuxian and lan wangji are married. from what jiang cheng hears, it was a small and private ceremony, with only family and close friends invited. jiang cheng, of course, was not invited. jin ling had been invited, but when he heard that his uncle had not received an invitation, he had declined to attend.
on the night of wei wuxian and lan wangji's wedding, jiang cheng somehow finds himself back at the cave. the incubus is still there. the incubus once again transforms to look like wei wuxian. jiang cheng and the incubus sleep together again.
time passes. jiang cheng continues to occupy his days with the running of lotus pier. however, increasingly, he spends more and more of his evenings at the cave, with the incubus that has transformed to look like wei wuxian. initially, he and the incubus only sleep together; however, as their arrangement goes on, jiang cheng finds himself doing more and more nonsexual activities with the incubus as well.
initially, jiang cheng only sees the incubus as a source of weakness, as a way for him to pretend he's with wei wuxian; due to this, all his interactions with the incubus are rather abrupt. however, the incubus - along with transforming itself to look like wei wuxian - has also mastered the art of acting like wei wuxian. just as wei wuxian used to do, the incubus jokes with jiang cheng, teases him, argues with him. just as wei wuxian used to do, the incubus responds to jiang cheng with happiness, amusement, or sadness and anger. and - just as wei wuxian used to do - the incubus promises jiang cheng to stay with him forever.
time passes, and jiang cheng gets attached.
jiang cheng begins to feel bad about the dismal conditions of the cave (which he gathers the incubus has chosen to make its permanent home), so he builds a small house for the incubus inside the cave and fills it with furniture. jiang cheng begins to feel bad about the incubus living in constant darkness, so he lines the cave with talismans that fill it with (timed) light. and jiang cheng begins to feel bad about the incubus having nothing interesting to look at, so next to the house he built for it, he builds a small pond and fills it with lotuses. (note that the expenses for all of this came from jiang cheng's own accounts, not yunmengjiang's finances.)
jiang cheng also begins to feel bad about the incubus being alone when he's not here, so he begins to take the incubus outside the cave. though the incubus is initially reluctant, it soon finds that, with a weimao on and jiang cheng by its side, it is fully able to pass as human.
all of these changes make the incubus happy. initially, it had only seen jiang cheng as a convenient source of yang energy, prey more powerful than it had ever ensnared before, and was mostly pleased with its own daring and outrageous success. however, as time passes, the incubus begins to enjoy jiang cheng's company, and finds itself treasuring even the meetings in which no yang energy is obtained. and, truthfully, the incubus has little need for material gifts or furniture - but the fact that jiang cheng found the incubus important enough to invest time and resources into procuring these items for it makes the incubus incredibly happy.
what makes the incubus happiest, though, is walking outside by jiang cheng's side - is being recognized by all the rest of humanity as jiang cheng's companion.
that is when the incubus knows that things have to change.
because, after all, humans are companions to other humans. humans, being social animals, mutually benefit when they enter into relationships with other humans, as is their nature - an incubus, however, has no such nature. an incubus's only instinct is to take advantage of this social instinct and steal from their hosts. because of this, a human who ends up as an incubus's prey has only one of two possible futures: either they are abandoned once the incubus has eaten its fill, or the incubus stays to eat until the human, completely depleted, dies.
and what other future can exist for jiang cheng and this incubus? will jiang cheng bring the incubus back to lotus pier, marry it properly, allow it to become jiang-furen? will the incubus by some magic give jiang cheng fully human heirs? will all of jiang cheng's beloved lotus pier accept the incubus without any protest? will this arrangement then continue forever - with no negative impact on jiang cheng at all, despite all the yang energy the incubus will need to take from him? impossible, impossible. from the start, the incubus knew it was all impossible.
after all, why was jiang cheng even with the incubus to begin with? was it not for the mere fact that the incubus could transform itself - could change itself to look like the human "wei wuxian"?
the next day, the incubus speaks to jiang cheng. the incubus tells jiang cheng that their relationship cannot continue; that, instead of clinging onto the past, jiang cheng should properly move on and find himself a real human partner - and that, in order to facilitate this, the incubus will leave the cave and never return. then, before jiang cheng can say anything, the incubus disappears.
the next time jiang cheng visits the cave, the cave is empty: everything jiang cheng built for the incubus is still there, but the incubus itself is gone.
In Cross Dimensional TFA, TFA Megatron actually likes the G1 version of Starscream compared to his Starscream but G1 Starscream hates his guts because that is still a Megatron and this Megatron stole a play from his book. I mostly thought this would be funny especially if Thundercracker accidentally leaves behind one of his datapads and Megatron picked it up and started reading his screenplays.