The most consistent trend I've seen centers on Venti's bard persona being a front for something much older and wearier, with Lumine as a kind of grounding force. Writers love playing with the idea that she's the only one who sees through the 'carefree bard' act to the ancient wind spirit underneath, which creates this lovely intimate tension. You get a lot of hurt/comfort where Venti is dealing with centuries of guilt or exhaustion, and Lumine is the pragmatic but gentle traveler who offers silent company or a shoulder to lean on. It's less about grand romantic gestures and more about quiet understanding, which fits both characters perfectly.
Another massive one is the 'bard and muse' dynamic, but flipped on its head. Instead of Venti inspiring Lumine, it's often her journey and her resolve that reignite something in him. Fics will have him following her across Teyvat, not as Archon or guide, but as a companion who finds his own purpose in her mission. This lets authors explore the world through a dual POV that's both whimsical (Venti's) and determined (Lumine's). The romance builds slowly through shared campfires, stolen glances during festivals, and Venti composing songs about her that she only half-understands.
Then there's the trope of 'contractual cohabitation' – which sounds formal, but it's usually something like Lumine needing a place in Mondstadt and Venti offering his attic, or them being forced to share a room during a festival due to a booking error. It's a classic setup for domestic fluff and gradual closeness. You'll see a lot of scenes with Venti trying to teach Lumine to play the lyre, or Lumine dragging a hungover Venti out of bed, that sort of thing. It leans into the found family aspect of the Traveler's journey while adding a layer of sweet, mundane romance.
A niche but growing trend I enjoy is fics that focus on their shared immortality, or potential for it. Lumine is an otherworldly traveler who may outlive everyone, and Venti is an Archon who has watched eras pass. Stories that pit them against the flow of time, where their relationship is a constant in a changing world, hit a different emotional note. It's less common than the other tropes, but when done well, it's profoundly bittersweet.