I last posted a theory about 1 year ago. Since then, we've had both Captain America 2 and Guardians of the Galaxy released as well as some pretty awesome information about Avengers 2: Age of Ultron itself. So I decided to throw my hat in the ring and attempt to put some pieces together of what i think/hope will happen.
Spoilers may lie ahead
just as a warning to those of you that want to go in fresh. But let's be honest, if you clicked on the link, you probably are into reading spoilers anyway. Here we go......
After the party in Avengers tower that we see during the extended trailer 1, everyone is sitting around relaxing having a few drinks. The futile attempts to lift Mjolnir are rudely interrupted by what seems to be the first appearance of Ultron.
What if this is a celebration of a successful operation? Let's say, oh I don't know, defeating Hydra at the Siberian(?) base. That mission may also be their first run-in with the twins. And as we all know, to the victor go the spoils. And the spoils here is Loki's staff.....housing the Mind Gem.
When placing the scepter in the lab maybe leaving it a little too close to an Iron Man Drone. A drone that The Science Bros had been experimenting on. Experimenting with the A.I. program call Ultron.
Being augmented with cosmic power from an Infinity Stone would expose most of the Avengers (sans Thor) to an enemy that has been directly imbued with this cosmic energy for the first time. This could explain Tony and Bruce's inability to understand the programming of this A.I. as well as its ever-adapting ability to stay alive.
The [Guardians of the Galaxy](movie:424073) are already aware of the Infinity Stones and Thanos. So the cut scene could be something along the lines of the Guardians getting some signal that there is an Infinity Stone on Earth (Terra), which can cue one of Chris Pratt's whimsical comments
Or better yet, let Avengers 2 show the Mad Titan himself and have him realize where the "final" stone is located. And have him set in motion his plans to begin collection of "his" Infinity stones.