In many ways the first Ant-Man film was a the perfect film to follow Age of Ultron - it was an origin tale that had smaller stakes and a more personal story yet still helps set up the next film(s).
‘Ant-Man and the Wasp’ needs to perform a similar role but will follow Infinity War Part 1, the biggest MCU film to date.
Act 1
Depending on the outcome of Civil War and the line-up for Infinity War Part 1 it’s possible we might not have seen Scott or Hope since the conclusion of Civil War. I would like to see them on their own, away from the Avengers and unaware of the events in IW Pt1. Instead they are forming a plan to rescue Janet from the Quantum Realm. To successfully do this they have been working with Hank who has re-designed his method for them to enter the realm but they need some new equipment. This means they need to steal it off a company/organisation such as AIM or Quinn Worldwide which is in turn controlled by Hydra. Hank knows about this equipment as his old business foe Elihas Starr aka Egghead developed it with him and then took the credit. However due to the complex security systems Lang needs help – and so we get Hope as the new Wasp, and a big action sequence with the pair.
Act 2
After the successful heist Ant-Man and the Wasp enter the Quantum Realm under the supervision of Pym who stays out to monitor them etc. This leads to an InnerSpace/Fantastic Voyage type adventure to find and rescue Janet. They eventually find both her and the Soul Infinity Stone. As Wikipedia says this “allows the user to steal, control, manipulate and alter souls, living or dead, and is the gateway to an idyllic pocket universe that is much like heaven. It also has the ability to steal the powers and abilities of others, which is why people argue that this gem is more powerful than the other gems. The Soul Gem is sentient and has a hunger for souls. It is often considered to be the most dangerous and corrupt of all the gems. Mastery of the gem would allow the user to control all life in the universe.” Elements of this can be used for the MCU version of the Soul Gem – it can explain how Janet has lived in the Realm for decades, hasn’t aged at the same rate, and has basically been a prisoner of the Soul Stone.
Act 3
Whilst they are rescuing Janet the company they stole off realise what they have taken and with Hydra’s assistance kill Hank Pym and steal his monitoring equipment, leaving the trio seemingly trapped in the Quantum Realm forever. However after some Janet flashbacks they adopt some of plans from an old 80’s mission and manage to gain control of the Soul Stone and escape with it back to the real world. After some more 80’s flashbacks they then stage a heist to get the equipment back and later combine it with the Soul Stone to use it’s powers to resurrect Hank.
Post-Credit
Either Dr Strange or Captain Marvel arrive, requesting the Soul Stone and the assistance of Ant-Man and The Wasps…