Firstly, let me say that while I personally prefer films that attempt to stick to the source material as closely as possible, I understand that there are times to deviate when it serves the overall narrative structure of a piece. Considering that Hank Pym is a complex character in his own right, and has yet to be introduced in the MCU, I feel that it would be a mistake to try to shoehorn him in without organically allowing him to have his own cinematic debut.
However, I also think that while it is quite easy to have Ultron without Pym, the bigger disservice is having Pym without Ultron. What I mean is, Ultron will be the same character with or without Pym, but unless Pym is Ultron's creator, Pym himself won't be the same character.
With that said, Marvel also has a rather good track record of painting itself out of the corners it's gotten itself into, (for example, "The Consultant" One-Shot as a way to explain the ending of The Incredible Hulk, which no longer made sense with the story they'd decided to tell, or the fact that Tony Stark was allowed to join the Avengers in a time of crisis, even though he had previously been given the boot from the Avengers Initiative in Iron Man 2) so the likelihood that, down the road, we find out Hank was involved in Ultron's creation is pretty high, in my opinion.
Now, my current working theories on Ulton's MCU origins, as well as how Hank Pym could be tied in later on down the line are as follows --
1.) Stark Creates Him
It seems to be popular consensus among fans that Ulton's creator in the Avengers' sophomore outing will be Tony Stark, but the how and why remain up for debate. I currently believe that, if Stark is indeed Ultron's creator in the Cinematic universe, then Ultron's origins will go a little something like this -- After Iron Man 3, Tony tries to give up being Iron Man to be with Pepper, and upgrades JARVIS to more effectively pilot his suits (as they didn't seem all that well-controlled when JARVIS took them over during Iron Man 3's climax), should the need for some heavy-duty heroics arise. This upgraded version of JARVIS is referred to as ULTRON, and eventually goes rogue, attempting to protect humanity by wiping out its greatest threat, coincidentally also humanity. This prompts Tony to don the armor once more and join the Avengers in dealing with his creation. This raises a question that's never been touched on in any of the solo Iron Man films, though... How is a glorified mechanic (as he self-describes in Iron Man 3) able to create such lifelike artificial intelligence? The easy answer is "he's a genius", but likely the more accurate answer can be taken from a throwaway line in The Avengers, in which Tony states that he became an expert on thermonuclear astrophysics "last night" simply by reading the files that SHIELD had provided him with. So, if Tony Stark can manage to, in the space of less than 24 hours, grasp something that complex, he could certainly figure out how to create his own artificial intelligence simply by reading research on the topic. Research written by...
(drum roll please,)
Hank Pym. Down the line, in the Ant Man solo film (if Hank is even in the solo film, as speculation seems to suggest that Scott Lang will be Ant Man, and not Pym), I believe that Pym will discover that his theoretical research had been used as the basis for Ultron, and will have some sort of existential crisis, believing himself to be responsible for unleashing such a monster into the world, and reinstating a much-needed "My God, What Have I Done?" factor to Pym's character.
The other option, however, I find more promising because of the fact that, according to Kevin Feige, Captain America: The Winter Soldier will be the most important Phase 2 film. This other option is that...
2.) SHIELD Creates Him
In the SDCC '13 Captain America: The Winter Soldier footage (which, admittedly, I wasn't lucky enough to see), we see Robert Redford's Alexander Pierce state that he joined SHIELD to protect the world, and that "to build a better world sometimes means tearing the old one down", which seems to be where he's coming from as far as sending SHIELD strike teams out after Cap and Black Widow. My theory is, after failing to take out the two Avengers by the end of the film, Pierce will commission SHIELD scientists to build Ultron as a way to deal with not only these two Avengers (and Falcon), but the rest of the team as well. Ultron would, in this story, be created using existing SHIELD technology, Chitauri technology, and potentially a mix of Iron Man's technology (obtained from all of those destroyed suits from the end of Iron Man 3) and whatever technology was used to create Winter Soldier's bionic arm. The team that is responsible for Ultron's creation will be virtually faceless in the Avengers sequel, but by the time the Ant Man solo film rolls out, Hank Pym could be identified as one of the scientists on this team, and he could begin to feel remorse for being responsible for Ultron's creation.
Regardless, I'm very excited to see how Ultron is handled on his first outing in the Marvel Cinematic Universe.
As always,
Excelsior!