So, I thought I'd give it a shot at "fixing" this movie. My own version of "What If?" if you will.
The movie starts off with Scott Lang in a good space: famous author, a stable family life, respected as an Avenger and recognized as a hero by everyone. (kinda like how the actual movie starts, then.)
He seems content, but inwardly, he's antsy. He used to be a thief, remember. Although he's left that life behind for good, a small part of Scott wishes he's still in the thick of action. And, with his middle age looming, so does his mid-life crisis. So, he wants to "retire" with a proverbial one last job, but his desire to catch up with his daughter prevents him from doing something reckless/dangerous/world-saving aka the usual Avengers stuff.
Scott's daughter, Cassie, questions why her dad isn't being more active toward helping people without knowing her father's feelings. (Again, kinda like the actual movie, but with a little more dramatic tension, I think.) She wants her dad to act like an Avenger, but little does she know, the Avengers haven't called Scott since the end of... uh, the End Game. Scott is inwardly frustrated, but his family comes first, so he bottles it all for now.
During the pizza scene (from the actual movie) it is revealed that Cassie and Hank have been working on a quantum realm-mapping-whatever thingymajigie. They go to the basement workshop and start sending the signal to the quantum realm, only to receive an urgent SOS message back from Janet's old acquaintance who is "trapped" down there. This acquaintance speaks of a "Conqueror" who's spreading a reign of terror in the quantum realm, and only Janet can stop him using her Wasp suit's abilities - by stealing a McGuffin in the villain's vault.
Janet exposition-dumps at this point, saying what happened during her time in the quantum realm, and says she owes this acquaintance and must repay the favour. Realising this might the last hurrah he's looking for, Scott volunteers. After all, he's a thief, and stealing world-saving McGuffin is his thing.
Cassie also wants to come, but Scott obviously says no, then asks Hank to keep an eye on her. Scott, Hope, and Janet travel to the quantum realm and meet the acquaintance, a scientist from the distant future named Nathaniel Richards (twist!) who self-exiled himself to the quantum realm for a past he wishes not to reveal. But he's revealed to be a good man, helping the denizens of the quantum realm with his knowledge and benevolence.
The "conqueror" is Krylar (played by Bill Murray, like in the movie) who used to be Janet's old flame while she was stuck in the quantum realm. (This might create another point of tension in the story. Especially when Hank learns about it--like in the actual movie, but handled a little better) MODOK is his minion but dreams of freeing himself from Krylar's influence somehow but can't due to some kinda device enslaving him. Or something.
So, Scott and his gang are dodging Krylar's paramilitary and working together with the resistance to steal the McGuffin... But Scott, as a thief, instinctively feels something is off about this whole thing but can't be sure what that is.
Meanwhile, up on the surface, Cassie convinces Hank via Talk-no-Jutsu ("Just don't be a dick, okay!") and they also end up in the quantum realm and straight into Krylar's hands.
So, the usual Marvel superhero things happen, CGI explosions go off everywhere, heroic speeches and tension-breaking banters ensue until they ostensibly steal the McGuffin. That's when Scott's thieving senses go off and he figures out why he's been feeling so uneasy: how did Krylar conquer the quantum realm?
Just before the heroes celebrate, ominous BGM starts playing, and Nathaniel Richards looks up in horror and says, "Oh, God. He's coming."
Janet asks, "Who is coming?"
N. Richards: "Me. My variant. Kang is coming."
A lone figure in purple-and-green suit slowly descends, his face obscured by a sinister helmet, and says, "That's enough playing around." A proverbial snap of his finger, and the entire battlefield is flattened.
In the last ditch effort to save his friends, Nathaniel Richards opens a wormhole with his tech, then tries to send Scott and Co. home before Kang kills him in full view of everyone and takes away his McGuffin that allowed Krylar to conqueror the quantum realm.
The gang tries to escape but Kang still interferes, and Scott and Janet are separated from the rest. The "rest" arrives back in San Fran, but Scott and Janet end up in another universe, stranding them there. (I'm thinking of Earth 838 from Doc Strange 2, which will connect the two movies.)
Cassie, Hank, and Hope swear they'll bring Scott and Janet back home.
Roll credits.
Mid-credit scene will still be the "Council" of Kangs, but they gather to talk about Kang the Conqueror assuming control of the quantum realm which will give him access to any and all part of the multiverse. They want to stop him from continuing with the multiversal war as mentioned in the Loki TV show.
Whew, now I'm exhausted.