One common theme is the exploration of their different backgrounds. Cyno from the desert and Tighnari from the forest have distinct cultures and ways of life, and fanfictions often explore how they bridge that gap. Another theme is friendship turning into something more. They start as comrades and gradually develop romantic feelings.
There's a really interesting Cyno x Tighnari fanfic called 'Under the Sumeru Moon'. In this story, they are on a mission together to solve a mystery in Sumeru. As they work side by side, their feelings for each other start to grow. It's full of mystery and a bit of action, which makes it very engaging. Also, 'Harmony of Elements' is a fanfic that shows how their elemental powers interact and how that affects their relationship in a unique way.
As of now, the Genshin manga is not finished. The story is still evolving and there's more to come for fans to enjoy. The creators are taking their time to develop a rich and engaging narrative.
There's no definite answer yet. It might continue if there's enough demand and the creative team has more story to tell. But it could also stop if they feel they've completed the story arc.
There is a Genshin Impact manga available. It brings the game's world to life on the page with beautiful art and engaging storylines. It's a great companion to the game.