Warning: This post contains SPOILERS for Ant-Man and The Wasp.
The post-credit scene for Ant-Man and The Wasp ties directly to the end of Avengers: Infinity War, but some fans think there's an even larger connection made. Just like with almost every Marvel Studios movie, their twentieth film ended with two post-credit scenes. The final scene was one final joke, but the mid-credits scene gave the connection to the larger universe fans were waiting for.
In Ant-Man and The Wasp's mid-credit scene, Hank Pym (Michael Douglas) and Janet van Dyne (Michelle Pfeiffer) are rigging up a new Quantum Tunnel. The purpose of the latest venture into the realm is so Ant-Man (Paul Rudd) can collect quantum healing particles for Ghost. The mission is a success at first with Ant-Man safely traveling back into the Quantum Realm, but on Earth, Hank, Janet, and Hope van Dyne (Evangeline Lilly) have vanished thanks to Thanos' snap. This leaves Scott alone in the Quantum Realm with no clear path to escape. While he's freaking out though, some fans believe there's a glimpse at another major MCU character.
A theory from r/MarvelStudiosSpoilers claims to see a smiling Thanos briefly. The main detail is what looks to be an eye form in the background, with a flash of light quickly revealing the Mad Titan's smiling face. Its tough to tell if this has any merit to right now based on the scene alone, but other details about Thanos could disprove this line of thinking. Recent concept art from Avengers: Infinity War revealed Thanos' farm is actually on Titan and possibly even in the past. While most theories on Avengers 4 point to the Quantum Realm being used by the heroes to travel across time, Thanos was able to just use the Infinity Stones (namely the Time Stone) to make his travels possible.
However, Thanos isn't the only purple villain fans claim to have seen; some argue that the eyes are actually that of Doctor Strange villain Dormammu, the ruler of the Dark Dimension. This theory could hold a bit more weight as both the Quantum Realm and Dark Dimension are alternate dimensions accessible through the Multiverse. Dormammu's been sworn to stay away from Earth, but he could theoretically be looking to conquer other dimensions instead.
As exciting as either of these appearances in Ant-Man and The Wasp's post-credits scenes could be, both are far from confirmed. The fact that fans are even debating over which MCU villain it could possibly be teasing goes to show just how unclear this all is. It may simply be a case of fans seeing what they want to see, finding faces in random patterns. The Quantum Realm is full of strange lighting and shapes, after all. There are indeed be teases of the MCU's future buried within the frames of Ant-Man and The Wasp's various Quantum Realm scenes, but this doesn't appear to be one of them.
At the end of the day, this is likely nothing more than a fun hypothetical. Could Thanos or Dormammu have used the Quantum Realm before? Absolutely. But is this possible Easter egg teasing anything more, if anything at all? Probably not. And for Ant-Man's sake, let's hope either of these villains aren't trapped down there with him.