WARNING: The following article contains spoilers for Marvel’s Ant-Man and The Wasp, in theaters now.
Walton Goggins' Sonny Burch is important to Ant-Man and The Wasp, because he's a more straightforward foe who allows Ghost (and, to a certain extent, Bill Foster) to be more of a nuanced antihero than outright villain. Once Ghost is cured, she's no longer a threat. Burch, however, has to be stopped because he's trying to steal Hank Pym's laboratory for nefarious reasons rather than the understandable rationale Ghost has.
Before he resorts to outright theft in his attempts to acquire Hank Pym's lab, Burch offers to pay Hank and his daughter, Hope van Dyne, a billion dollars for it on behalf of a mysterious third party. The film doesn't reveal who the mystery buyer is, but we can make some guesses.
One of the unresolved stories from the first Ant-Man film is that one of Hydra's agents managed to escape with some Pym Particles. The idea was that Scott would recover the Pym Particles in the mid-credits sequence, but Marvel Studios decided to instead tie in with Captain America: Civil War, so the scene where Scott got the particles back landed on the cutting-room floor. Still, it is probably technically considered "canon" that Scott got them back from Hydra.
If not, then this could be a perfect way to bring Hydra back into the Marvel Cinematic Universe. Yes, the organization has been defeated a number of times, but come on, Hydra's slogan literally is "Cut off a head and two more shall take its place!"
Advanced Idea Mechanics
Advanced Idea Mechanics, or A.I.M., was essentially an evil think tank founded by Aldrich Killian. It was one of the main bad guys in Iron Man 3. However, by the end of that film, Killian was dead and his empire was in shambles. They didn't explain, however, whether A.I.M. survived the death of Killian.
A.I.M. is a mainstay of the Marvel comics universe as an evil science group, so the organization would make a perfect buyer for Hank Pym's lab. In fact, in the comics, Hank has run afoul of A.I.M. on a number of occasions, including once when it pretended to have his seemingly dead first wife as a captive.
The MCU is well known for how few villains make it out alive after their initial appearance. Luckily for Justin Hammer, he was only the secondary villain in Iron Man 3. He hired Ivan Vanko to replicate Tony Stark's arc reactor technology. However, Vanko used the tech himself to become a supervillain. Hammer ended up in prison for his role in the whole mess.
However, he is likely out of jail by now, and for a guy who is all about ripping off other, smarter minds, it would be perfectly logical that he would try to take advantage of a chance to buy the life's work of the brilliant Hank Pym.
An intriguing theory regarding the mystery buyer might be that the reason that his or her identity is kept secret is because Marvel Studios does not yet have the rights to the character. In other words, this could be part of a multi-film attempt to slowly introduce a mystery science villain and then reveal that it is Doctor Doom behind it all!
The villain's rights currently belong to Fox, but if the merger with Disney goes through, Doom's rights will once again be part of Marvel Studios' collection of characters. Doom even has history with shrinking, as he invented a machine that could shrink people to nothingness. It was accidentally turned on himself and instead of dying, he ended up in the Microverse, as we met that world for the first time.