I would love a prequel trilogy set in the late 70's and 80's. Here are some thoughts:
Call the films the Avengers Initiative
First film set in 1978-ish
Pym has already developed his ant-talking tech and just worked out the last kink on Pym Particles. He is a hermit and pacifist. He prefers his ants and science to people and their troubles. In comes a Jan van Dyne, a fashion model/designer who takes an interest in Pym. Pym isn't buying it and uses his suit to follow Jan and finds she is a SHIELD agent using fashion as a cover. Jan is there to recruit Pym to SHIELD but used the cover to get closer. Pym is recruited as a scientist but refuses to give up his tech to SHIELD. Because of this, it is decided Pym will retain his technology and be trained as a SHIELD operative by Jan. Because of his pacifism, Pym is difficult to work with but Jan is determined and works with him to put his abilities to use. Pym eventually loses his pacifism and beats the snot out of an evil-doer. It is decided more heroes would be needed to handle larger threats and protect Pym from himself.
I'm not familiar with the comics so villains are unknown. I kind of like the Tinkerer as a Pym confidant and mentor with Grizzly as his henchie. Suggestions welcome.
Problems: Jan is a supporting character. I would like to get her in a suit but Pym wouldn't trust her until after they have a relationship. The only way to fix this is to make Pym and Jan a couple from the outset. Jan as a bubbly, silly person is not leadership material. I don't think Pym would go for that personality anyway. This is personal preference but a tougher Jan, not too different than her daughter Hope, seems to me like the best use of the character.
Several years later...
Film #2 1983-ish
"There was an idea, Stark knows this. Called the Avengers Initiative. The idea was to bring together a group of remarkable people, see if they become something more. See if they can work together when we needed them to. To fight the battles that we never could."
-Nick Fury
"PHASE 2 isn't ready, our enemy is. We need a response team."
"The Avengers Initiative was shut down."
-Nick Fury and the World Security Council
Fury knows all so he would know of any past uses of superheroes by SHIELD. He would know their history and how they are necessary to preserving our way of life. But how would he know that a group of remarkable people would work? Well, he didn't but he had evidence that it could because Peggy Carter did it first.
The first Avengers team was formed after Jan van Dyne went to SHIELD leadership and urged a recruitment drive of powered people. With Director Peggy Carter's blessings, Jan van Dyne began identifying and recruiting powered individuals to combat threats that conventional soldiers can't. Jan van Dyne was the perfect choice to lead this group as she is an exceptional strategist and fighter working for SHIELD.
Van Dyne's first recruit is Hank Pym who calls himself Ant-Man. Pym does not want to share his technology with anyone fearing the nefarious use of his scientific achievement. Jan has been working with Pym since the first film. Trained by Jan, he can hold his own but prefers to avoid a fist-fight as he is prone to rage.
The second recruit is Union Jack. Growing up hearing tales of heroic feats performed by Captain America during WWII, James Falsworth's son Brian trains and hones his abilities to fight against those who would do the world harm. Carrying his trademark dagger and pistol, he is a deadly shot with any firearm. He is an exceptional athlete but not super in any way. His suit is bullet-proof and stab-proof after upgrades by Howard Stark. Brian Falsworth develops the alter-ego Union Jack and gets on SHIELD's radar after taking down a small HYDRA cell in Britain.
The third member was in a crate, locked away until Howard Stark reactivated him. The Human Torch is an android whom we first saw during the first Stark Expo. Stark bought the technology and has been working on perfecting it for 30 years. Torch is an android, but much more clunky than Vision. He is more robot than man and it shows. He can fly and control flame. He is strong but not indestructible. He takes orders. This is a dumbed down version of comics Torch. He can think a little but is limited by the tech of his time.
The fourth member would come in after the first group mission is a bit of a catastrophe. Jan van Dyne tried to coordinate the fight from distance but found that she needed to be more hands-on. Over the years working with Pym, Jan and Pym had grown close and married. After they married, Pym began work on a personal suit for Jan to join him. Pym named the suit Wasp and gave it to Jan. As undisputed leader, Jan decided the team needed a name and settled on the Avengers.
Who do they fight? I don't know. HYDRA guys or something. I imagine seeing a young Nick Fury as head of STRIKE. Howard Stark and Director Carter should be around. If HYDRA is the villain, the thrust of the story could be their infiltration of SHIELD meaning Zola would have a role. The Tinkerer could be recruited to SHIELD after the first film but he turns HYDRA too. Where's Winter Soldier? He's a ghost but he should be active, working against our heroes but avoiding direct confrontation.
The real problem I'm seeing with this is it doesn't give a lot of time to the development of these new characters before we see them together in action. I don't think we need to see Human Torch much, he's easy and everyone will get him, a short introduction will be all he needs. Union Jack could be cool and there is definitely a story there. Captain America inspired a lot of people, but why haven't we seen someone dress up and fight crime? Now we will!
Film #3 1989
We know the ending, in 1989 SHIELD sends Ant-Man and Wasp to intercept an ICBM headed for the United States. Jan goes sub-atomic during the mission and is lost in the sub-atomic world and presumed dead. Pym, in his grief, retires from being a superhero and ends his relationship with SHIELD.
The film starts with the team together, everything is dandy. Pym and Jan are married, Jan recently had a baby, Hope. HYDRA wants to eliminate our team, they are too much of a threat. HYDRA creates a trap with a missile drawing the Avengers in to destroy them. Human Torch is destroyed or severely broken, Union Jack takes down the HYDRA leader while Jan and Pym go to stop the missile where Jan "dies".
HYDRA set up the Avengers to fail. Jan, the leader is gone. Pym, her husband is distraught. Torch is scrap and Union Jack is left going "WTF". Fury could have been leading a STRIKE team on this missile base and saw how important this team was spurring him to resurrect the Avengers Initiative later when he was promoted to Director of SHIELD. Fury may be going WTF seeing the team destroyed.
This could be made into a HYDRA wins trilogy which would be completely different than anything else Marvel has done to date.
These are just ideas and a lot of bare bones. Obviously this is my idea so I like it and I hope others agree it would be fun to see something like this in the future.