The upcoming Ant-Man film will focus on the second Ant-Man Scott Lang, who is mentored by the original Hank Pym. Here's the trailer in case you haven't seen it yet...
In this trailer we get clues as to what Hank Pym has been doing for the last 40 years with regards to the [Ant-Man](movie:9048) suit and Pym Particles. I believe that a sequel to Ant-Man would best be a prequel starring a young Hank Pym, inventing the suit, going on Adventures and fighting the bad-guys of his day (either HYDRA or A.I.M.)
We could even have 60's/70's era cameos featuring Agent Peggy Carter and a middle-aged Howard Stark (played by Dominic Cooper) as well as a glimpse of the 60's or 70's era S.H.I.E.L.D.
This might be a good vehicle to showcase Villains like Baron Zemo or MODOK who would have been running things post WWII for HYDRA and A.I.M. respectively.
I would like to see Patrick Wilson cast as the young Hank Pym/Ant-Man
He's already played a Superhero before (Night-Owl from The Watchmen) and he looks like a younger Michael Douglas.