What would've happened in the illusion saga if dark steve returned instead of the demon steve
I'm actually not familiar with Dark Steve. I didn't watch any of his episodes fully, so I apologize if my ideas aren't as developed as my other AU ideas. Nevertheless, I hope this answer was worth the wait!
Anyways, we know that Dark Steve is a shapeshifter, just like Illusion Steve. And Dark Steve tricked many Steves in the past by disguising himself as an Elder Blue Steve.
So, having two shapeshifters in the same arc would be confusing as heck. I think Dark Steve can do some pretty malicious things to confuse both the heroes and Illusion, and Dark Steve can do even worse after Reality's failure of a plan.
Like, what if Dark Steve somehow hijacks Illusion Steve's plan, basically stealing all of Illusion's work and making himself the evil ruler of the new world?
Sabre might try to talk to Dark Steve to make him stop doing this stuff. Maybe he'll tell Dark about Nightmare Steve (how he turned good and that Elemental's the reason why Nightmare was evil in the first place). Will Dark Steve have a crisis, knowing that his creator/boss is dead and had a change of heart? Will Dark wonder if Nightmare regrets creating him? Or will Dark think Nightmare is a disgrace and a weakling for turning good, and Dark will become ashamed that he was created by such a weakling, becoming more determined to do evil things?
Illusion might also try to talk to Dark Steve. They're both disastrous messes, after all. Illusion might offer Dark a brand new purpose. Or maybe Illusion and Dark will both relate in thinking that their creators are disappointments.
I know you were asking about Dark Steve in the Illusion arc, but can you imagine what would happen if Dark Steve was in the Ender Crystal arc???
Like, imagine if Sabre successfully convinced Dark to stop being evil and they just team up. Imagine if Sabre brought Alex and Dark on his quest to save the Spirit World. (Dark agreed to come along because he's really curious to meet good, reformed Nightmare in the Spirit World).
And then, imagine the awkwardness and saltiness that'll happen when Hypno Steve joins the group, aka team up with Dark Steve, the Steve who created his miserable existence. And if Dark is reformed, then meeting Hypno would not just be awkward, but super duper awkward.
We would have episodes-worth of awkward/salty Hypno and Dark banter.
Anyways, that's all the ideas I have for this AU. Thank you for the ask.