While the Quantum realm is an important part of the plot Ant-Man and the Wasp, we don't actually see that much of it, but it's possible that wasn't always the case. Recently, artist Aleksi Briclot revealed some of his concept work for Ant-Man and the Wasp which showed a very different version of what Janet Van Dyne would look like when we met her near the end of the film. However, what's potentially more interesting than the look itself, is what inspired it. According to Briclot, at one point there was consideration given to the idea of finding an entire civilization living inside the Quantum Realm.
When we actually see Janet Van Dyne inside the Quantum Realm for the first time she looks like a nomad who's been wandering the desert for the last decade or so, which, in many ways, isn't far from the truth. It's not clear where she got her new threads since she went into the Quantum Realm in her Wasp costume, but hi-tech is not exactly the word to describe her when we see her again. However, it could have been, as this image shows Janet wearing a much more technological outfit, which apparently would have been created for her by a civilization that actually lives inside the Quantum Realm.
Of course, just because we don't see a civilization inside the Quantum Realm in Ant-Man and the Wasp doesn't mean it doesn't exist. It could be that seeing it was removed from the movie, it feels a bit superfluous considering that's not what the movie ended up being about. We do see things that appear to be life forms that exist there, so a civilization certainly could in theory. There's so much the Marvel Cinematic Universe has yet to explain about the Quantum Realm in the first place.
Of course, there's a good chance we'll learn a lot more about the Quantum Realm in Avengers 4. Based on the situation that Ant-Man has found himself in post-snap, the details of the Quantum Realm could end up being very important for him, and by extension, the rest of the surviving heroes.
While this particular look for Janet Van Dyne was apparently scrapped because they decided to go a different direction with the Quantum Realm as a whole, there's another reason that it was probably a smart movie. It makes Michelle Pfeiffer look a lot like Calhoun from Wreck-It Ralph. Although, now it makes me want to see Michelle Pfeiffer star as Calhoun in the live-action remake.
While it's often believed that Marvel movies end up getting locked into particular plot points in order to serve the larger narrative, it seems that exactly what the Quantum Realm is, and what's inside it, are not necessarily set in stone.