With Rainbow's return to Equestria, a great many things changed, both for her and Equestria. Celestia and Luna - Sleipnir's daughters by Clover and Pansy respectively - were officially Asgardian Aesir, although they'd never contacted Asgard or asserted the inheritance that was theirs through their paternal grandfather, whose adoption into the Asgardian royal family had never been revoked despite his status being stripped from him for his crimes. However, with Rainbow being named a Warrior of Valhalla - based in whatever worlds she called home - Equestria's relation to Asgard had to be reconsidered. Given that, officially, it was ruled by Aesir, its technical status was either that of a colony or an allied nation. In either case, it opened the door for Equestria to request - and receive - a certain amount of Asgardian magitech for their use.
However, all Celestia and Luna requested was that which Tony Stark would need to build a stable portal between Equestria and Earth. Because of the level of raw magic Equestria had readily available, the Tesseract wasn't even needed to power such a portal. Once it was established - at the top of a new Stark Tower built in Manehatten, given where the balance of the magical field allowed for easiest and most stable controlled spatial distortions - Equestrian envoys immediately began negotiations for an alliance of equals between Equestria and Earth, with the exchange of magic and technology to allow both worlds to grow. With a new alien race walking peacefully on Earth amongst its citizens, many conflicts faded into history within a month. Celestia then arranged with Nick Fury on behalf of Earth's leaders to place a petition before Odin to the indication that Earth had outgrown its protectorate status.
Odin began drinking again.
Rainbow's personal life, on the other hand, was a bit more complex. She had returned with Twilight and the others to Golden Oaks Library in Ponyville to talk about her two lives. While she had some difficulty adapting to the idea she was a jock, her own internal drive to 'be the best' was constant between her two lives, and she'd been able to find the connections between herself and her friends of her previous life. While there had been some initial awkwardness, it felt to her as though she had put on an old dress that didn't quite fit right anymore; she just needed to get used to it again. For some reason, that metaphor had made Rarity squeal happily.
After discussions of how she could better adapt to her old/new life - which resulted in a rather heated debate of comparitive theoretical sciences, magicks, and physics with Twilight that went way over everyone else's head - it quickly became plain that Rainbow would, barring a few new quirks, slip back into her Equestrian life without problems.
As events passed, she grew more and more used to Equestria, especially as it changed with new events...
Applejack glanced up at the armored figure catching her as she fell through a trap door in the old Palace of the Pony Sisters. "Rainbow?" she said in shock.
"I gotcha, AJ," Rainbow replied, pulling her back up and setting her down.
Applejack caught her breath, then glanced at her companion. "We said no armor for you for the 'Most Daring Pony' competition," she complained teasingly.
"Yeah...figured your safety was more important than winning," Rainbow admitted as she put the suit away.
Thinking about that, Applejack grinned. "So let's call that bit a mulligan. Care ta keep going?"
Rainbow grinned widely. "You're on!"
As Rainbow and her friends helped to stop Ahuizotl, she wasn't sure which was more awkward for her: discovering that one of her favorite story book authors was also the main character; or that said author/character desperately wanted Iron Mare's autograph.
Rainbow Dash stepped back from Mane-iac as the spasms wracked her body. Turning, she saw her friends staring at her in shock. "What?" she asked, surprised. "I trained with Thor. Lightning and storm powers aren't exactly new to me."
"But she was monologuing!" Spike complained.
"I know! Left herself wide open!" Rainbow tilted her head in confusion when Spike facepalmed as they were drawn out of the enchanted comic book. "What?"
Rarity's competition for fashion week in Manehatten went off without a hitch. Her primary competition, Suri Polomare, had lost her assistant not long ago to a talent scout working for Stark Industries: Manehatten, who felt she could utilize her talents to a much greater degree working as Pepper Potts' personal assistant. For some reason, this resulted in Miss Polomare's work in the fashion industry taking a steep decline, which led to all sorts of speculation. She also hadn't been able to get - or keep - a new assistant since.
Rainbow's performance in the relay race tryouts at Rainbow Falls for the Equestria games proved phenomenal, exceeding even the expectations of those who knew her, despite not being able to use her armor. However, she politely declined Spitfire's request to fill in for Soarin' after the Wonderbolt stallion got injured, stating that it wouldn't be right to fly with the Wonderbolts until she, herself, was a Wonderbolt. Beyond that, Cloudsdale was no longer her hometown: New York City and Ponyville were the only places that qualified as that now, as far as she was concerned.
Rainbow had at first eagerly volunteered to help take care of Discord when he got sick - especially after it was revealed that he was Sleipnir's brother - but proved too easily distracted with trying to analyze the blue flu...and later the green flu he acquired from being sneezed on by the Tatzlwurm.
Rainbow could only stare in stupefaction at the "Goof Off" that would decide who would plan her 'Birthaversary'. While she had initially pushed for Pinkie Pie and Cheese Sandwich to plan the party together - Pinkie planning the birthday related activities, and Cheese planning the anniversary related ones - that had gone out the window when a third party planner threw their party hat into the ring.
She had expected the craziness Pinkie would come up with for this competition.
She wasn't too surprised at what Cheese Sandwich brought in, either. She was familiar enough with Weird Al Yankovich to recognize an alternate manifestation when she saw it.
...She did not know Nick Fury had such a good singing voice...
It had been awkward enough trying to bond with Pinkie Pie's sister, Maud, when she came to Ponyville to visit, since she was so different from Pinkie and hard to relate to.
Discovering that she was dating Ultron did not help.
When Rainbow Dash introduced the study method Tony had used to teach her history to Twilight, the young Princess had found it absolutely fascinating. So fascinating and engaging, in fact, that Rainbow nearly missed the History of the Wonderbolts exam she was studying for because they got so into the lesson.
However, she made it just in time thanks to the boost her armor gave her, and passed the test with flying colors.
...she still wasn't sure why she was trying to get into the Wonderbolts Reserve. She was pretty sure being an Avenger and a Warrior of Valhalla was a lot cooler than that. Then again, growing up with Tony had taught her that there was no such thing as too much cool...
Rainbow was briefly upset when she found a dress suddenly magicked onto her. However, she was able to get out of it by activating her armor. She then spent the rest of the day helping others deal with Ponyville's sudden 'fancification'.
"Wait," she asked Twilight at the end of the day, "you mean this wasn't a normal part of the festivities?"
Twilight Sparkle nodded as Celestia finished informing her about Tirek. "Don't worry, Princess Celestia! I won't let you down! I'll track down Tirek-"
"Actually, Twilight, I had someone else in mind for this task," Celestia interrupted. "Discord."
Twilight blinked. "Your uncle?" she asked, feeling deflated.
"Yes," Celestia explained. "He has the ability to detect the disruptions in the magical field created by Tirek stealing magic-"
"Had, actually," Discord interrupted. "Do you have any idea the level of disruption that Rainbow Gate has created? My head's been buzzing with its signal from the moment it was built! I still haven't figured out how to filter it out."
"So...you can't track Tirek down?" Luna clarified.
"Not easily, no," Discord replied.
"Twilight, stay near your friends," Celestia instructed. "If we can't catch Tirek off guard, we need to make sure we aren't. It will be some time before he's strong enough to drain an alicorn..."
In Ponyville, Tirek had swelled to immense proportions. He had managed to catch the town off guard, and had caught hold of Rainbow during her nap. Before she could activate her armor, he had begun draining her and her arc batteries, which gave him enough power to return to full strength, standing as tall as the Library. "I know not where you learned to store so much magic in such a way," he gloated, "but I thank you! You have made my conquest so much easier!" He paused as he sensed the magic in the medallion Rainbow was wearing. "What's this?"
Rainbow grinned, punching the medallion. A rainbow shot down from the sky.
Hulk stepped out of the rainbow, dressed in his Mrs. Doubtfire outfit. He gasped as he saw Rainbow's muted colors and missing Cutie Mark. "Who hurt Hulk's little pony?" he demanded angrily.
Tirek quickly tried to hide Rainbow behind his back, whistling innocently. Hulk was frightening normally. Angry while crossdressing, he was downright terrifying.
"Bad centaur hurt Hulk's little pony!" Hulk roared. "Hulk Spank bad centaur!"
Rainbow blinked. "Don't you mean smash?" Rainbow asked in confusion.
"No!" Hulk proclaimed. "Hulk Spank!"
Hulk then grabbed Tirek, pulled the massive centaur over his knee, and proceeded to spank the stolen magic out of him.
This was the scene Twilight came across as she returned to Ponyville to gather her friends in case of an assault from Tirek. She stared, head tilted, trying to make sense of what she was seeing. She couldn't.