I may be a little late for debating about the need of having the Pym in future Avengers film... but I just would like to point out a few things.
I really, really like to see Hank and Janet in Avengers 2, but maybe this wil never happen...
In this case, Ultron as the villain is nevertheless something that could very well happen in Avengers 2.
They could make a simple change that a many of the fans wouldn't like, but that wouldn't be very much of a problem in my opinion: let Tony Stark build Ultron instead of Hand Pym. BTW, Tony was Ultron's creator in one of the animated films.
While such a change wouldn't be true to the comics, it has a certain logic. The movie's Tony Stark is the world's leading expert for artificial intelligence, robotics etc.
Instead of being a robot, Ultron could be an artificial intelligence computer programm at first - the next version of JARVIS. And as JARVIS II/ULTRON has complete control over Tony's manufacturing machines, it could use them to build himself a nearly undestructible robot body, using Stark's armors as a blueprint. It would make a lot of sense in my opinion.