My favorite t4t wangxian headcanon is that Lan Zhan is a trans man and Wei Ying is nonbinary but he comes out after they get together.
Wei Ying grows a lot when he gets to college, meets and befriends more queer people than he ever has before, and learns a lot more that he didn’t even know there was to learn. Somewhere in here he starts dating Lan Zhan and tries drag and starts asking himself, well, what makes me a boy anyway?
He eventually has a conversation with Lan Zhan that starts, “How did you know you were a boy?” except that instead of being invasive he genuinely wants to know because he thinks that the part of him that is a boy might be...missing. Like, he’s been thinking and searching around inside himself and it’s just not there? He’s just been conforming to how everyone sees him and it’s fine but what if...what if something else would make him happier?
So Wei Ying and Lan Zhan talk it out and experiment together and eventually Wei Ying comes out as nonbinary, mostly agender, but they have fun with gender presentation (it’s like trying on a bunch of different costumes!). They’re constantly experimenting and having fun and that...that brings them joy.