It's totally gonna be Tony Stark who makes him; there's no one else who it would make sense for. Heck, even if Ant Man had come out before Avengers 2 it would've made more sense for Stark to make Ultron. There's just more emotional connection to him. Only way Pym would've worked would be if Ant Man was a phase one film.
also from that issue of EW:
Spoiler: Stark's Masterplan (click to see content)
For better or worse (trust us, it's worse), his Tony Stark has devised a plan that won't require him to put on the Iron Man suit anymore, and should allow Captain America, Thor, Black Widow, Hawkeye, and the Hulk to get some much needed R&R as well. His solution is Ultron, self-aware, self-teaching, artificial intelligence designed to help assess threats, and direct Stark's Iron Legion of drones to battle evildoers instead.
The hard part about battling Ultron, as the cover image suggests, is that he's not just a robot—he's a program, capable of uploading himself and disappearing not into the clouds but the Cloud. And he has a bad habit of rebuilding himself into stronger and more fearsome physical forms.