A strong BL fantasy manuscript needs a world readers want to explore and a relationship they want to follow. These elements should strengthen each other rather than compete for attention.
Begin with the central promise of the story. What makes this world distinctive? What brings the characters together? What stands in the way of the relationship? A clear premise helps readers understand why this story needs to exist.
Worldbuilding should affect the plot. Magic, politics, history and social rules need enough consistency for readers to understand the consequences of character choices. The manuscript does not need to explain everything at once, but it should avoid rules that change only when the plot needs them to.
The romance needs its own arc. Attraction is a beginning; trust, conflict, vulnerability and choice create development. Both characters should have agency and meaningful goals.
Finally, consider pacing. Give readers time to understand the world, but keep the story moving through decisions and consequences. A compelling BL fantasy balances emotional intensity with a plot that gives those emotions somewhere to go.