A/N: I'm kind of a fan of Loki/Tony. This is my idea of how their storylines might have crossed and a relationship formed there, pre-Avengers movie. Starts with Iron Man storyline, then Iron Man 2 and parts of Thor overlap – liberties have been taken and gaps have been filled. Oh, you'll see as we go along. Looks like this is going to be a long one. Also – angst, because these are two characters full of issues.
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Loki is young when he learns of the pathways, the hidden routes to other realms that negate the need for the Bifrӧst. Thor, older and worthier, is finally granted permission to travel the realms at will, sometimes alone but most often with Sif on their "adventures". Frigga tells Loki it is his age, nothing more, that restricts him to the relative safety of Asgard. Loki bites back his bitterness – nobody ever takes him seriously standing next to his big brother – and tries to believe her. He tells Thor he has no desire to go on their foolish quests anyway, he's busy, he's learning things, and people can do that when they aren't idiots. He spends his time in the deepest, murkiest corners of the most forgotten of libraries, immersing himself in mysterious looking scrolls and heavy-bound books buried in layers of dust. Amongst these, he finds the most intriguing of tomes; bound in a leather of jewel-purple, calligraphical and interspersed with hasty scribbles in multiple hands. Loki spends sleepless nights reading every word and realises he's found answers he didn't know he was looking for.
Of course, finding his way to Vanaheim without the Bifrӧst is not, it turns out, the same thing as hiding from Heimdall. Having traded for a few magical artefacts – trinkets at best, nothing more, Loki is eyeing a much more interesting stall when he looks up and sees his father storming towards him. Odin takes him home in a disgrace he has trouble feeling over the thrill of his (limited) success. He insists he shouldn't be punished because he has no idea how he came to be in Vanaheim and since Heimdall has already admitted he didn't see the Prince travelling, his claim goes unchallenged.
After that initial success, Loki thoroughly researches the process of full concealment. Invisibility won't work on Heimdall, he knows from attempted tricks of his childhood. There are few mentions of hiding from the all-seeing even in the most promising of texts. Those he finds describe only failures. There's potential in a few methods, though, and Loki sees them as compatible, working on a way of overlapping two failures with a simple spell of his own creation. His solution makes him feel different, but testing it is another matter.
Thor attempting to restrain his laughter is always an amusing sight, especially when combined with Odin's anger. Thor's face is bright red, his shoulders shaking while he bites his lip, as Odin demands that Heimdall explain how his most prized stallion came to have extra legs. Heimdall's surprise is genuine as he explains he saw nothing untoward. Thor mistakes Loki's smirk for supressed amusement rather than the triumph it truly expresses. Loki has won. The victory is small, as he uses his trips to other realms merely to trade for items Thor always seems to forget to bring back for him. It is the principle of the thing that pleases him; Thor is not as superior as he likes to believe.
Before long, Loki is joining his brother on the trips he takes to other realms, with his new friends in addition to Sif. Their adventures are still foolish, chasing magical weapons or mythical beasts unlikely to exist in anything other than children's tales. More often than not, they find a kind of trouble they weren't expecting. The others, warriors born, will enjoy any kind of battle. Loki struggles to find most of their fights stimulating. More often than not, their opponents lack any kind of tactical knowledge or common sense, meaning Thor gets the upper hand within minutes. From time to time, they encounter bands of mercenaries, who are usually cunning and an even match for them. Loki truly enjoys those battles, using his mind rather than learned skills. He's often invaluable to them, when they face opponents with brains, but the others would never admit that. He always has a sense he doesn't fit in. The Warriors Three are suspicious of his magic and Thor could never understand his fondness for the staff and his throwing knives. Lady Sif always takes his side in such disputes, arguing time and again the need for grace and skilled restraint. In time, the arguments are less harsh, words more teasing than wounding.
He uses his particular method of hidden travelling only for his journeys to Midgard. Odin doesn't approve of too much interference in that realm any more, and it becomes somewhat of a guilty pleasure for him. Loki admires humans most of the time, can't look down on them the way others do. He certainly enjoys what they call technology and spends large amounts of time there investigating televisions and computers and their various vehicles. Many of them are stupid, but he encounters enough idiocy in his own realm to know that's no indication of an entire species. He supposes the Warriors Three would find it hilarious if they knew he also slept with some of them, but he discovers that they are more than capable of keeping up with those they once deemed gods.
Loki befriends Tony Stark by accident. He's at some kind of ball or gala or some such event, all formal dress and champagne when Tony Stark happens to be flirting with the same girl he has his eye on. This becomes a ridiculous competition of wits and smug expressions until the poor girl hardly knows if the sexual tension is based around her or between the two men. She flees with another man and Tony feigns annoyance for a moment before flashing a grin and offering to fetch Loki a drink. Loki's mildly surprised when he actually returns, a drink in either hand. They're honestly intrigued by each other and by the end of the night they are making drunken pacts of friendship, laughing, hugging. Loki's drunk enough to tell Tony he's a traveller from another dimension, a sorcerer, a Norse god. Tony's drunk enough to believe him.