The entirety of Till Forever Falls Apart by Ashe and Finneas is very Wangxian vibes but the "I’d spend a lifetime giving you my heart, I swear that I’ll be yours forever, Till forever falls apart" seems very them in particular if you wanna go for it
Lan Wangji returns to the Jingshi to find his husband practising sword forms outside. His footsteps slow until he is just standing on the path, watching Wei Wuxian go through the movements, sure and steady. He’s only wearing two layers of robes, sweat sticking his hair to his face. The sword is trembling slightly in his hand, but not as much as it used to, when he first took back Suibian from his brother.
Wei Wuxian turns and catches sight of him. He lowers the sword, a grin at the ready, waving his other arm. “Lan Zhan! Welcome back!”
“Wei Ying,” Lan Wangji greets. He walks the rest of the way to his husband, unable to take his eyes off the flushed cheeks, the air of contentment and weariness from exertion of effort. Wei Wuxian looks healthy. Alive. Absolutely glowing.
“Ah, no, I’m all sweaty!”
Lan Wangji ignores the half-hearted protests, leaning into kiss Wei Wuxian. “How long have you been practising?”
“About one shichen. I joined the juniors in the training hall in the morning. I’ve really built up this body’s stamina, huh? Aren’t you glad, Lan Zhan? Are you thinking about how our everyday is everyday will be even better now?”
“Making love with Wei Ying is always and will always be good,” Lan Wangji says just to make Wei Wuxian blush. Truthfully, he hasn’t been thinking about how their sex life can be improved, or about how they can further research dual cultivation. He’s thinking about Wei Wuxian flying alongside him on his own sword when they travel, about Wei Wuxian clashing swords with him on a moonlit rooftop again. “I am glad Wei Ying is enjoying training.”
Wei Wuxian shrugs. He wipes his wrist under his chin, eyes drifting towards the gentians in the garden. “I’ve still got a long way to go, to build up this core. But I’ll do it, you know. I’ll catch up to you. Lan Zhan will wait for me, yeah?”
Lan Wangji follows his sweaty husband inside. He sneaks in another kiss under Wei Wuxian’s messy ponytail, tasting the salt of the heated skin there, before going to draw a bath. He listens to Wei Wuxian undress behind him, to Wei Wuxian’s lively chatter. To the windchime Sizhui gifted them from one of his travels south, tinkling on their doorway. To the soft breeze rustling through the bamboo behind the building. He wants this forever, he thinks. Coming home to his husband, living through season after season after season with him. A lifetime will never be enough. If they should cultivate to immortality, Lan Wangji knows he will never grow tired of growing old with his Wei Ying.