What we've learned from Problem Slueth is that the more insignificant and meaningless a nonentity or mute character is, the more important it will be. Added points to ones that someone has irrational emotions toward. For example, the Ethnic Mural that Sleuth loves earlier on than is suddenly terrified of, becomes an army of monsters he fights. Ace Dick hits some random Game of Life peg that looks like a woman. He is for some reason devastated by this and begs for her life. She lives and Ace then marries her. When she is gunned down later on, it raises DMK's emotion meter hugely, and turns out to be crucial to his defeat. The best one of all is Captain Snoop. He is just a random bust sitting on Kingpin's ship. When he is launched off he lands on a small, floating isle. When it drifts off, Problem Sleuth reaches out then lowers his arm sadly, suddenly deeply attached to it. Below it says "I'm sorry. You have to know when it is time to say goodbye. However, Captain Snoop's island is destroyed and he falls onto the flying legs of a candy mecha, smashes into the moon, drifts down to the Sleazy Brothel in the Sky, is blown off of the legs by a huge fan whose current had just finished travelling across the universe twice, falls through a giant puzzle cube floating in the sky, has the universe turn upside down to change his trajectory, gets sucked into a black hole, dodges the beams of a demon shooting lasers at demigoddesses, and smashes into DMK's weak spot, defeating him once and for all. And this all happens to some random bust of a rapper sitting on a ship's deck. Yeah, I bet you're going "wow" right now.
Anyways, I'm certain something like this will happen in Homestuck, possibly multiple times. List dumb things you think will be important! Here are my three:
Creepy Wolf Head: it magically appears anywhere that something extremely dramatic is going down, and it apparently summons lightning and darkens everything around it to give a spooky effect. Andrew is also terrified of it and also included it in the alive/dead character image with a frowny face drawn over it. If anything were to be a game changer against one of the most powerful antagonists, it's that.
Captain Snoop: He blocks Dirk's door. He was a hero once and I'm sure he'll be one again!
Dear, Sweet Casey: John loves her like a daughter and Rose assigns Casey as her familiar. I wouldn't be surprised if she has some hidden power that she releases at the critical moment.
I can think of others, but don't believe in them as much. Tell me your ideas now, and what you think of mine! ^ω^