Consider how the properties of red-black trees (coloring, black-height) must be maintained after merging two trees where all keys in one tree are less than all keys in the other.

Explore how to efficiently combine the two trees while preserving the BST property, possibly by treating one tree as a subtree of the other and adjusting colors/pointers accordingly.

Investigate whether a linear-time merge is possible by leveraging the red-black tree's balancing properties, or if a more complex restructuring (like rotations) is required to maintain the invariants.