The Homestuck Disc Portfolio depicts a series of dots representing the Acts. Four red dots for Acts 1-4, a blue dot followed by a red dot for Act 5 Acts 1 & 2, a green dots for Act 6, and a white dot for Act 7. We already know that red represents the Beta session, blue the Trolls' session, and green the Alpha session. Since there is only one green dot, it seems that the Alpha session's story will be resolved in one Act. White is something outside those three universes, or possibly a combination of all three. Since Lord English exists in multiple universes, Act 7 is most likely centered around him. Plus, since he's the Big Bad, it only makes sense for the final act to feature him.