Had originally started this sometime last year and abandonned it because i was entirely happy with basing it on the Iron Man armour. However after Marvel's recent announcements about Avengers: Age of Ultron i feel that this could be the direction they might go.
It appears to me that Marvel might have Tony Stark create Ultron instead of Hank Pym, which i hope doesn't turn out to be true. However i think it would be good to link Tony Stark, and more specifically JARVIS, into how Ultron is created.
My idea would be to have Hank Pym working on a new AI for SHIELD, called Unified Logistics, Tactics, Resources and Organizational Network. The system helps co ordinate their troops and weaponry in the event of large scale to global events, and is a reactionary measure by the World Security Council to the Battle of New York. . It inevitably becomes self aware, and corrupts JARVIS. It then uses the designs in JARVIS' databases to create an armour for itself, and possibly create an army of drones. ULTRON also integrates advanced weaponry into his armour using designs from SHIELD's databases. Two notable examples are the Phase 2 weaponry developed from the captured HYDRA technology, as well as the reverse engineered Destroyer weaponry.
This would avoid the potential problems of how Hank Pym could create the ULTRON armour, given that Iron Man 2 established how far behind the rest of the world was from creating 'Iron Man' technology.