Even asleep, Al could tell this dream was different.
He sat beside his brother in a deep lecture hall, and the lecture was all about extremely combustible fuels and some kind of nozzle and someone named Goddard. Nii-san’s hair was longer than in any of the pictures Al had from those lost years, even the oldest ones, and he was dressed so respectably Al wanted to stare.
But he couldn’t.
Al leaned closer to Nii-san, which he hadn’t intended to do, and murmured "So? Do you think this one can make it?"
They weren’t Al’s words. They weren’t his thoughts.
"Maybe," Nii-san murmured, eyes fixed on the lecturer. "But making it out of atmosphere is just the first step."
Al laughed–only he wasn’t–and spun a pen between his fingers. "Ed, you’re obsessed."
"I’m getting back," Nii-san stated, with determination that Al remembered in a way, from other dreams. And then he glanced over, mouth curling wryly. "Besides, you’re just as obsessed with your own stuff, Alfons."
Al’s start woke him up.
And that was when I got stuck for a little while, and other series intervened, and I eventually admitted that I wasn’t going to finish writing this.
How it was going to finish: Al would realize he’d found Ed in an alternate world by dreaming contact with his alternate self, and head back to Izumi with his insights to figure out a way to bridge the worlds and retrieve his brother. Gil would, with a little prompting from Amos, realize that he really has bonded deeply with Al and would go with him, and there might even be a kiss somewhere in there to get Al to understand why Gil wants to travel with him. Al is entirely pleased with this and shows Gil off to Izumi, much to Izumi’s amusement and Gil’s exasperation. Gil and Sig bond over the trials of being an alchemist’s lover while Al and Izumi get as far as stabilizing Al’s contact between worlds while he’s waking, but can’t figure out how to get someone through without them getting caught by the fragment/creatures in the Gate.
In the meantime, Ed would be researching furiously on how to get to another world via space, and Hohenheim would be sneaking around trying to stop a Nazi experiment in human sacrifice involving the mass death of asylum inmates. Hohonheim would, of course, be captured, and Ed would chase after him, bursting in on the scene just in time to stop the bad guys, but not in time to save his father. Hohenheim would alter whatever inscription/array the Nazi would-be-magicians were using to make it actually work, and sacrifice himself to open the Gate and send Ed through, where of course he promptly enters Al’s perception and is dragged back home on the strength of Al’s knowledge of what the Gate truly is.
(I don’t actually remember if I even knew what I was planning to do with Alfons; maybe he meets a nice mechanic and falls in love.)
Once home, Ed would be delighted to see Al whole, threaten Gil on older-brother-principles, be appalled to find out that Mustang had entered politics but glad that Lisa is keeping an eye on him, and be happy for Winry and Rose that they’d made a match of it. He and Al would bury themselves in theory for a while, hammering out what it was that happened to them, and what it means about the universe. Eventually, though, Al would drift more toward the philosophical end, and he and Gil would return to Ishvar while Ed took up his travels again, looking for fellow alchemists who would listen to the new theory and also for trouble.
The end.