I've had some thought about this, and thus I have a few ideas about the powers of various heroes. I'll go with The Avengers for the moment:
Captain America: Several months of training, surgury to alter the musculature and tendons, and taking the right combination of preformance enhancers could, in theory, make someone into a real life Captain America.
Iron Man: Hell, they've been working on strength enhancing exo-skelitons for years, and recently I saw a video of a working demonstartion of jet boots. All you really need to to is improve upon and combine the two, and you've got yourself an iron clad avenger.
Thor: The strength levels are possible if you use the Captain America method plus the instilation if high powered servo motors on the joints and metal reinforcement of the bones. This would make the subject much heavier, but he;s meant to be a tank anyway. The hammer is also theoretically possible. Manipulation of the weather could happen through a manipulation of electyromagnetic feilds, but the hammer would have to be quite large and it would only operate on a small scale. However, with the subject's strength, the size of the hammer wouldn't be an issue. I doubt flight through use of the hammer would be possible, but a specially designed cape with glyding capabilities and jet propulsion, like the flight harness recently created by the U.S. military, might be doable.
Hank Pym: There realy is no practical way of growing or shrinking someone like that. However, the manipulation of insects is somewhat plausible. If the person in question wore a helmet that emited the proper frequencies of EM radiation, or possibly emmited pheramones, then it stands to reason that he could control insects. Also, while he could not physically grow himself, the possibility of using a mech is not out of the question. However, such a device would be mighly expensive.
The Wasp: I'd say a suit with wings and taser like stingers like hers are possible, although I'd think the wings would require too much feul to be practically worn on someone's back.
Hawkeye: He's just a guy with arrows. The only difference between him and any professional archer in the world is that the professional archers don't fight crime.
Black Widow: Just a lady in black leather with guns. Not hard.
Quicksilver: A combination of the Thor and Captain America procedures, but concentraiting entirely on speed. Doubtful that he'd get to Quicksilver's level of speed, but he'd still be damned fast. However, it's quite likely that such a procedure would take too much of a toll on the subjects heart, resulting in his death.