This is a post about the post-credits scene in Ant-Man and the Wasp, aka the very last scene in the movie, aka this post is FULL OF SPOILERS.
So by now you've seen the whole movie and you know that not only can our heroes go subatomic (aka get microscopically small) and safely return, but the Quantum Realm is also full of healing energy with untapped potential.
Dr. Janet Van Dyne (Michelle Pfeiffer) explained that 30 years in the Realm changed her, and likened the alterations in her body to human evolution (so it doesn't sound wholly negative). We only got a quick taste of her new abilities when she got back and healed Ghost (Hannah John-Kamen) with some sort of energy that she transported through her hands.
In the first post-credits scene, we saw Janet, her husband Dr. Hank Pym (Michael Douglas), and Hope/The Wasp (Evangeline Lilly) transport Scott/Ant-Man (Paul Rudd) back to the Quantum Realm to collect more healing energy (or whatever it is) to continue treating their "new friend" Ghost.
Unfortunately, the events of Infinity War caught up with the Ant-Man timeline right after Scott went subatomic, and the ENTIRE Pym/Van Dyne family fell victim to Thanos' mass murder and turned to ash, leaving Scott stranded in the Quantum Realm.
WOW, things got very dark very fast. So let's unpack this, shall we?
First and foremost, the scene confirmed that the events of Ant-Man and the Wasp take place right before/during the events of Infinity War — which explains why Scott was absent from the movie.
Secondly, even though things kinda ended on a cliffhanger, it's very unlikely that Scott will stay stuck in the Quantum Realm for all of Avengers 4 — and even if he does, he'll still likely play a big role in the action.
And that "really interesting story" the directors have planned for the two Dad-Avengers could also end up being really sad. It's pretty much a mathematical certainty that not every member of both their families survived Thanos' attack.
But, as we've theorized to death (haha), it's almost certain that most (if not all) of the major players killed off in Infinity War will somehow be resurrected through the events of Avengers 4.
Maybe the mysterious new energy from the Quantum Realm will play a role in defeating Thanos. If Scott stays stuck in the Realm for some of Avengers 4, he might be forced to figure out how to help his friends from the other side.
The Quantum Realm might tie into the mysterious Soul Stone other-realm we briefly saw in Infinity War.
The scene also means that the Wasp won't be appearing in Avengers 4, which is a huge bummer.
And, lastly, I'm pretty sure the second post-credits scene of the ant drumming after everyone died was just to lighten the mood a bit.