I found myself endlessly annoyed at the lack of thought that went into most crossovers for these two fantastic universes. With all the pseudo-science in both, a lot of fun could be had with it, so I decided to take a shot. I'm not a great writer, so this won't be very long. One or two chapters at the most. Allons-y!
The moment Loki was successfully led away by Thor and some SHIELD agents to be chained, the Hulk's form shrunk down into a shivering Bruce Banner. Bruce, having dealt with two transformations in a single day, promptly collapsed against Tony, unconscious from exhaustion. The billionaire stumbled awkwardly in his depowered suit, nearly falling over himself.
Giving up the effort of standing while supporting heavy armour and another man's weight, Tony sunk down to sit comfortably in what he now was dubbing the 'Loki-crater' in the floor of the top level of Stark Tower. He was at least careful to set Bruce down in an area relatively void of glass, before activating the manual removal sequence on his suit.
Following their cue, the other Avengers seemed to collapse where they stood. Natasha slumped against a burnt settee along with Clint, and Steve gingerly sat down cross-legged, leaning against the bar.
"I think I'll keep the crater," Tony tried to joke, but it fell flat. It was actually pretty comfortable, as far as floor-craters went; and it was a decent place to drop bits of scorched armour in when removing the complex system of his suit by hand.
Steve glanced out over the edge of the tower, where he could hear sirens beginning to blare. Rescue efforts for people trapped under collapsed buildings and God knew what else were beginning.
The group of four gave collective inner groans. Steve was the first to get up, and it only took a glance between them for his meaning to be understood. As exhausted as the remaining Avengers were, they were still able-bodied enough to help rescue efforts. Tony eyed Bruce's unconscious form.
"Oi, Cap, put the Green Giant here on the couch before you go."
At Steve's look he added, "Hey, I'll come down to help too, just give me time to get out of this," he gestured to his armour, only partially dismantled, "and make a few calls. Do some major donations to rescue efforts," and give Rhodey an earful for not showing up, but that went unsaid.
Actually, best get on that now. "Jarvis," Tony tried, eyeing the general areas where he'd installed the speakers. Steve meanwhile, was carefully lifting the unconscious scientist.
Jarvis' voice system was damaged, but it managed, "What-t are y-your wishes, sir?"
"How's the elevator?"
There was a ding and the door opened. "Op-perational, sir," Jarvis put in. Tony felt mild annoyance, but at least something still worked right. The other three made their way inside, Steve giving Tony a final grim nod before the door closed.
Tony sighed before speaking to his AI, "Jarvis, see if you can contact Pepper again..."
They'd been helping at different points in the city when the army finally rolled in. Tony, Natasha, and Clint seemed content to leave it to them, but it had taken several civilian volunteers, the NYPD, and the temptation of a free meal courtesy of the local East-Indian joint before Steve finally stepped down.
Thor had returned to Stark Tower and picked up an exhausted Bruce before meeting them there. They had shoved two of the three remaining tables together and were just starting in on their ordered shawarma when the door opened to a young couple, unusually exuberant for the chaotic environment.
"Brilliant business, still running after all that," the tall man with spiky brown hair said with a grin, looking around. If the blonde woman's union jack shirt wasn't an indicator, his distinct English accent certainly betrayed their foreign origins.
The blonde gave the group a lookover, sharing an excited look with her companion. They took their seats on the remaining table, ordering a lamb shawarma and two samosas. They kept sending sidelong glances to the group as if waiting for something. The blonde made to say something, but the male shushed her and they giggled.
"Know something we don't?" Tony couldn't resist asking.
"Yes, actually, quite a bit," the man said easily, "Just waiting for you to ask. I'm the Doctor."
"Rose Tyler," the girl added, "and we really need to think of a new name for you, Doctor."
"Hm? Oh yes, well, I suppose," said the Doctor, "I'm going to have to go by John Smith now, won't I?"
"Not John Smith!" Rose exclaimed, "You get a chance to call yourself anything you like, you can't go with 'John Smith.'"
"Well, what would you suggest?" the Doctor, or was it John, turned toward Rose. They seemed to be ignoring their audience.
After a moment, she replied, "How about Don Noble? You're part Donna now, even if you didn't get the ginge part."
"Oi! Watch it," John, or Don apparently, looked scandalized, "Plus it's only part. John Noble?"
"Don Smith," Rose decided, "Flows better. Rose Smith sounds better then Rose Noble anyhow."
Oblivious to the implication, Don or John or whatever replied, "Why do you get to decide, eh? It's my name."
"Well, pick one. I swear, if you don't, I'll start calling you 'Handy'."
"Oi!" Tony repeated mockingly, "Whoever you are, Don or John or Doc', you said you knew something we didn't."
"Oh, yes!" the Doctor Donna-John turned toward them again, "Sorry. You're Tony Stark. Iron-man."
"Ooh, let me," Rose cut in, "Thor, the Norse alien god; Captain America, real name Steve Rogers; Natasha Romanov, also known as Black Widow; Bruce Banner, who becomes the Hulk; and Hawkeye, real name Clint... um... Beaton?"
"Barton," her partner corrected, gratefully taking his sandwich from the waitress, "Right on all other accounts. In other words, the original Avengers!"
The two beamed at them, looking thoroughly impressed. Rose took a bite out of her vegetable filled samosa. They now had the Avengers' full attention.
"How did you know that?" Natasha questioned, "The Avengers Initiative was top secret. Even after all this I doubt the press has uncovered that much."
"Is it?" Smith or Noble looked apologetic, "Whoops, a little early then. Must be the Chitauri invasion."
"How we know isn't important," Rose covered, "We can't even interfere until we know a little more about this universe. Brush up on our history, if you will."
"You speak in riddles," Thor proclaimed, "Speak your purpose, if not of yourselves."
"We have a letter to deliver," the enigmatic man said, pulling the letter from his jacket pocket, "and a ride to give. And we sort of can't do it without you."
"Second test drive, this is the only way we'll get it right for sure," Rose added, though no one understood her meaning, "I think I can live with John Noble."
"I'll add a Wilfred in the middle for her gramps," newly-dubbed-John seamlessly replied. He reached over the table awkwardly to hand the letter to Steve.
Steve tore open the letter and read it with an incredulous expression while John dug into his shawarma.
"What is this?" Steve asked, an angry and hurt tone in his voice, "and how is it in my handwriting? I never wrote this."
Ignoring Clint who had exclaimed a flat 'what?' and grabbed the letter, John explained, "It's a letter from your future self. You just created a fixed point."
"Meaning we have to do exactly what the letter says or we will end up creating a paradox," Rose chimed in, "and you really don't want to do that. Universe exploding business, that is."
"You missing a date with a girl could explode the universe?" Clint flatly asked Steve.
"But I can't do that. I'd have to, well, go back in time or something to get there," Steve added angrily.
"Well it's a good thing we happen to have a time machine in need of testing," Rose leaned back in her chair, "otherwise, the universe would be cooked."
"Time machine?" Tony and Bruce asked simultaneously. They glanced at each other before Bruce gave the go-ahead for Tony to say, "but that's completely impossible."
"Humans have no knowledge of Asgardian methods of transport through the Yggdrasil," Thor put in, "I'll admit, I have not heard of such a thing, but surely we cannot claim it to be impossible."
"Weelll, technically the technology doesn't exist in this universe," John elaborated, "We were actually in a different universe, building the TARDIS-"
"That's our ship. Time And Relative Dimension In Space, though I guess we have to add 'universe' into that mix somewhere," Rose continued.
"Alternate slash Relative Dimensions. TAARDIS?" John tried, drawing out the 'a' with an unimpressed look on his face.
"Anyway, we used a shortcut to grow the core, and it ended up being more explosive than we were expecting," Rose looked a bit guilty, "Bam! Here we are, new universe."
"That doesn't explain-" Tony started.
"The point is, that we don't actually know much about this universe yet, and it's impossible for us to pilot through anything but conventional space until we can lock on to at least two fixed points," John interrupted, "Then I can create an algorithm around the relative temporal distance between them."
"So... once you have the algorithm, you can safely travel to any time or space in this universe," Bruce finished hesitantly.
"Exactly," John beamed.
"How'd you get your first point?" Tony asked.
"We were in the future a bit," Rose answered, "Well, I say a bit, a few hundred years really. It's how we knew who you were. Original Avengers, very famous."
"Couldn't do much there, so we just punched in random coordinates and allons-y! Here we are!" John wiped his mouth with a napkin, done his sandwich.
"What's this all have to do with me?" Steve asked. All the talk about relative temporal distances and time and space algorithms went way over his head.
"We landed in Cardiff about six hours ago," Rose explained gently, "And we met a woman whose mother was named Margaret Turner. Maiden name Carter."
Steve could only stare in shock.
"She gave us the letter, and told us where to meet you," Rose finished, "and here we are."
"So how about it," John offered with a grin, "care for a trip?"
Taking Steve back in time is a cliche, but a fun one. Incidentally, shawarma is alright but samosas are what it's all about.