That's a good idea. I haven't done a big theorypost in a while, and I've been thinking a lot about recent events, and how they all fit together. I think I'm starting to get a sense of where we're headed.
First, let's recap the main events that we know will happen.
John will come into his own, achieving something that causes him to be recognized as a particularly successful Player.
Some disaster will eventually befall Rose, masking her from Trollian's viewport.
Jade will enter the Medium, alongside her volcano, the Forge. Her prototyping will empower Jack to his final form, likely including a dangerous ingredient such as Dream Jade, Becquerel or Typheus.
The kids will perpetrate The Mistake, a disastrous event which sabotages the troll session.
Both teams will collaborate on a plan which involves The Scratch, a Sburb phenomenon allowing the kids to tear holes in sessionspace.
John believes this plan will allow him to meet Karkat in person.
Based on this information, I think the end of Act 5 will revolve around the kids - John, in particular - opening the Scratch. This will be the Mistake, which brought the Demon to the troll session - but in true Homestuck fashion, the event which dammed them will also end up saving them.
I theorize that the rifts opened by the Scratch will lead to some neutral location. The kids can flee from Perfect Jack, the trolls can escape the Demon, and they can meet in person to decide on their next move.
I think it's here where Rose will find the Green Sun. By this point, she's already been obfuscated from Trollian - Kanaya implied this happened during the session - and we don't have a clue what this means, or what her game really is. It'll finally be time for her friends to confront her.
But I don't think Rose or the Horrorterrors are the real threat here.
It's Doc Scratch. It was always Doc Scratch. This phenomenon is even named after him, for god's sake. He's been manipulating the trolls, both in and out of their session, and he's got his grubby little hands over all their plans.
Scratch has been working to summon Lord English for centuries, and Aradia has described this sequence of events as the road to the undoing.
This whole clusterfuck will end, somehow, in Snowman's death, and Act 5's finale will begin the tale of Lord English, the universe-eater.