ANT-MAN 2: THE RETURN OF JANET
One thing teased at the end of the first Ant-Man was that Janet Van Dyne may still be alive within the Microverse. So the idea with the sequel would be to have Hank and Janet suit up with Scott and Hope. This would be a great way to bring Janet back and have her be a hero once again.
You would want to bring in a villain worth fighting, I think revitilizing AIM would be a great idea, and while originally an Iron Man villain, I would like to see Ghost brought in to fight Ant-Man. His stealth and hacking elements would make him a formidable opponent for Scott; and his backstory could easily be re-written to work with Pym Technologies and so on.
ANT-MAN 2: THE SECRET AVENGER
At the end of Captain America: The Winter Soldier, I think many fans started to see the potential in a Secret Avengers film. My belief is that if they were to ever do that it would be concealed as a character film, much like how Civil War is technically a Captain America film. The MCU being in a (probably) messed up state after Civil War and Infinity War, I hope that the public will go to distrust superheroes and The Avengers, and that Fury would call on Scott to lead a small team of heroes as the Secret Avengers.
The villains could be numerous, and with it being a sequel to Ant-Man, I would like to see Scott try to balance his newfound duties as a superhero with his duties of being a father.
ANT-MAN 2: SLAYING MANTIS
I can't think of a great title for this one but what I would love is that now that Scott has the costume, have many thieves see the suit as the ultimate get.
I'm thinking they could introduce William Cross as a leader of a group looking to bring down Hank and Scott for what happened to his brother, Darren Cross. Have him put out a bounty on Scott and Hank, and during this time intoduce Eric O Grady as one of Cross' people sent in, but when he gets a newer prototype of the suit, Hank and Scott now have to deal with Eric having the suit but with no responsibility. Ultimately you can have Hank, Scott, and Eric all teaming up to take down William Cross.
What do you all want to see in Ant-Man 2? Let me know in the comments below!