This is a great question, because (specifically the modern "XKit Rewritten" for the new Tumblr interface) has already solved many classic problems. However, Tumblr keeps changing, and new pain points emerge.
Here is a for XKit that addresses a current, annoying gap in Tumblr's native functionality: Feature Name: "Thread Weaver" (or "Reply Raker") The Problem it Solves: When you reblog a long post with a long comment chain (especially discourse or art critiques), it becomes impossible to follow who is replying to whom. Tumblr flattens all replies into a single chronological list at the bottom. If 5 people are having 3 different conversations in the notes, you have to mentally reconstruct the threads yourself. xkit tumblr