For a long time the following quote has been the only description of the Ant-Man plot; "The script for Ant-Man has been written by Edgar Wright and Joe Cornish, who plan to include Henry Pym and Scott Lang as major characters, with Pym as Ant-Man in the 1960s in a style similar to 'Tales to Astonish', and a flashforward to Lang as Pym's successor in the 1980s or '90s"
A lot has changed since the idea for Edgar Wright's Ant-Man was put forward but I think the basic idea should be retained; Pym in the 1960/70's and then Lang in the modern day (around the same time as Avengers 2).
The basic plot set-up could be something like this;
In the late 1960's Dr Henry Pym and his girlfriend Janet Van Dyne are working at S.R.R/S.H.I.E.L.D where Pym discovers the subatomic "Pym particles". This can lead to the main story of the film, a big period Sci-Fi/Spy adventure with Ant-Man and Wasp. After this is concluded the pair decide it is too risky to pursue the life of a Superhero, shelve their uniforms/equipment and return to safer lab work - in particular concentrating on Pym's 'Ultron' concept...
Then, in the present day (around the same time as 'Avengers 2') Scott Lang can steal the Ant-Man suit and equipment from Pym's home and we can have a small adventure with him. Although Lang is a thief it is revealed after he is caught by S.H.I.E.L.D that he had good intentions. Because of this Pym does not press charges and a friendship develops between the now 70 something year old Pym acting as mentor to Scott Lang the new Ant-Man, and a future member of the Avengers....
Some pros for this approach;
Although 'Captain America: The First Avenger' had modern day bookends this would be the first MCU film set in two different time periods. I'd imagine the 60's/modern day split to be roughly 70/30. This would place the bulk of the story between the first 'Captain America' and 'Iron Man' films in the established MCU timeline, a time period as yet unexplored on-screen.
In the comics Ant-Man and Wasp were founding members of The Avengers. Although this isn't the case in the MCU, having them working for S.H.I.E.L.D back in the 1960/70's could allow them to have an early role in the MCU. You could even have them as key early members of S.H.I.E.L.D – perhaps helping the transition from the Strategic Scientific Reserve (S.S.R, as seen in 'Captain America') to S.H.I.E.L.D.
It could lay groundwork for future MCU events e.g Ultron being started by Pym in the 1960/70's and then the research & development carried on by Pym and S.H.I.E.L.D over the decades. This could lead to appearances of Ultron as a villain in 'Ant-Man 2' (or even a future Avengers film).
In 'Iron Man 2' Nick Fury states that he knew Howard Stark, who was a founding member of S.H.I.E.L.D. Both men could have cameos in the late 1960's/early 1970's section of the film.
You could have fun Easter Eggs such as early concept designs for the Helicarrier, or set up upcoming films e.g mention that Wakanda is closing it's borders...
We could see some of the history of how the Strategic Scientific Reserve became S.H.I.E.L.D.
It could explain the whereabouts of the Cosmic Cube between being picked up by Howard Stark at the end of 'Captain America' and then appearing with Nick Fury at the end of 'Thor'.
Setting Pym's story in the 1960's avoids having another genius scientist (we already have Stark & Banner) in the modern Avengers team.
A chance for different relationships to be explored in the MCU; Pym and Van Dyne as an established couple in the past and then an elderly married couple in the present day, and also the master/apprentice relationship between Pym and Lang in the present day.
The introduction of Lang would bring in a different type of hero; one who is neither given powers or the inventor - he instead starts as a thief.
They could have a cameo of someone in the Ant-Man suit in 'Avengers 2' but not reveal until the Ant-Man film (released after 'Avengers 2') that it was in fact Lang, in the process of stealing the suit...