I think the Avengers are having a picnic? Maybe it's one of their days off. Anyway, Loki appears, but not to battle. He tells Thor that Doom has been making advances and reminds him that as Loki's older brother, it is Thor's duty to deal with him? I think warn him off and decide whether or not Doom is an appropriate candidate for his baby brother's hand. Or something along those lines.
Thor of course jumps to the chance to be brotherly with Loki and goes immediately to Latveria to possibly tenderize Doom with Mjolnir. Tony stays with Loki and flirting ensues.
Does it ring any bells? Please? I'm fairly certain that it was somewhere on Ao3.
FOUND: How Twilight would have gone if Bella had had an overprotective Asgardian brother by melonbutterfly