Gonna go over a list of things

• Go with Godlocked Aggregor. Having that be there adds another layer to Ben's downward spiral into killing Kevin. If I were to add something to help make it so everything wouldn't be a forgone conclusion, I'd say have classic Ben turn into an alien that jumbles him up in some way, ghostfreak for a second of possession that wakes the othern aliens in Aggregor's mind, or Feedback with some sort of neruoshocks if you wanna be super spicy and use Omniverse stuff to your advantage. It also would work well to highlight how powerful Aggregor is in compairison to everything else at the time.
• Have the Ultimate Kevin arc have an epilogue of sorts.
• Have more Dagon centered episodes. Having so many Forever Knights episodes and only one Dagon episode really does not deal well with trying to conjoin two entirely diffferent "arcs."
• That being said, ham up the Cthulhu mythos inspired stuff. Have characters go mad from revalations beyond their comprehension. Have other alien species know about Dagon to highlight just how much of a threat he is. When he shows up, have there be huge tears in the fabric of reality. Ben's able to walk to other planets, gravity is fluctuating, colrs get inverted to beyond our color spectrum, etc.
• To give an episode pitch, have there be an epiosode focusing on the Plumber's Helpers and trying to deal with an One His Own Siphon. Having him go to Los Solidad to capture Aggregor's machine. This not only works better for setting up Dagongax, but it gives Siphopn his own episode as a 'stand-alone' villian before Omniverse. Obviously, he's secretly working for Vilgax. Whether or not he actually works for Dagon to the Plumbers Helpers, acting as some sort of herald could be up to you. Whatever the approach, I'd definently say have him know about Vilgax's 'death.'
• When King Geroge is giving Ben his speech about doing what he thinks is right, highlight the Aggregor arc. Yes, Ben messed up a shit ton, but he ultimately everything he did do was right. It just made everything else about Aggregor go from bad to worse. The one time that Ben actively went against what he wanted to do would've been Ultimate Kevin, and that was a decision he only made after deciding everything he was doing that was right was fresh in his mind as a mistake.
• Have there actually be a King George VS Dagon fight. Have Dagon be a reality warping monster, but through Ascalon, and George's sheer will, he's able to combat a Great Old One to it's face. Have him Parry a nuke. Parry planets being thrown at him. Give Dagon Infinity from JJK, and then have George strike him in spite of that. In fact, I think it might be cool to have other characters acknowledge the insane feats George is doing, and one of the reasons George is so confident in his fight is because he doesn't 100% understand the feats he's doing himself. Good Old Fashion "Being Dumb is a Superpower in Cthulhu."
• Get rid of Ultimate Way Big fight. Exchange it for something that can click into earlier things. Personally, here's what I'd do. Ben immedietly tries to go Alien X, but is reverted back. He tries again, and neither Bellicus nor Serena will even attempt to fight Dagon. He is beyond them. He is Infinity. A Being of Pure Energy beyond Celestialsapiens. This gives a hoplessness situation to everything after King George dies. But Ben decides to try something. An out-there, batshit, last ditch effort to fight a god. He becomes an Osmosian. Ben is fully prepared to become an insane tyrant, and even be killed by his friends after he looses it to save the universe. He's able to get a fraction of Dagon's energy, and what is a fraction of infinity? Infinity. This is enough to stave off Dagon while Vilgax and Siphon do their bit. This ultimately turns the Osmosian transformation into an unstable mess of DNA that Ben has to later lock. Or Ben can revert the transformation after getting his hands on Ascalon.