Extremis is the basis of the threequel.
For those who are not familiar with the movie term, MacGuffin (not McGyver), it is a movie term coined by Alfred Hitchcock and it is a plot device used to propel a movie forward to its conclusion. The Indiana Jones franchise loves to use a MacGuffin and Marvel has used one as well. The Tesseract was the MacGuffin of Phase 1 in Marvel's cinematic universe since it was first introduced at the end of Thor and it carried over into Captain America: The First Avenger and into The Avengers.
Now that the Tesseract is in Asgard's protection, it could be replaced with another MacGuffin (Infinity Gauntlet?). Right out of the gate at the start of Phase 2, Iron Man 3 may be introducing Extremis as the next MacGuffin that could carryover throughout some of Marvel's other movies.
Here's where we get into some potential SPOILERS for Iron Man 3. You've been warned.
Some clarity has been brought to the attention of The Daily SuperHero via the new Iron Man 3 LEGO sets. This clarity is the fact in two of the new sets, based on the new film, are two figures that have been infected with Extremis. In the Malibu Mansion Attack set, there is an Extremis soldier (rumored to actually be the character Eric Savin) flying the Mandarin's helicopter that attacks Tony Stark's home. Also in the Sea Port Battle set, one of the movie's antagonists Aldrich Killian is infected by Extremis. Both of these LEGO figures have a glow-in-the-dark head because they are infected. See the images below, click to enlarge.
Sometimes toy manufacturers like to change their movie toy sets a little but since there are two Extremis-infected figures in two different sets, it appears this will be seen in the actual movie. And when infected with Extremis, these guys might appears to have some kind of super strength of some sort being that they (especially Killian in his LEGO set) will be able to stand their ground against the likes of Iron Man and War Machine.
So the question needs to be posted, is Extremis really an attempt to recreate the Captain America super soldier serum? In the comics, many have tried to replicate the serum used on Steve Rogers, with terrible results most of the time, and Extremis could be the movie version of this as well.
Or maybe is Extremis part of a bigger story that will carryover into other Marvel movies? Perhaps, our opinion post saying that Extremis could be the start of the villain Ultron's artificial intelligence development—helping set up 2015's Ant-Man movie.
One thing is certain and that is Extremis has a much bigger role in Iron Man 3 than originally thought and it may have lasting effects across multiple movies.
Iron Man 3 hits theaters on May 3.