It could be really good or it could be a clusterfuck.
The Age of Ultron is also the name of a company wide crossover that's going on in Marvel Comics right now. The gist of it is that Ultron is somehow able to take over the world in the future through some sort of thing going on with The Vision. When it comes to time travel and parallel universes getting mixed in with primary continuity, it can get confusing.
However, while I was typing this, an epiphany did suddenly strike. Age of Ultron for the film might be a completely different plot for the 2nd Avengers movie. Aside from the film's subtitle and the title of the crossover being identical, they might wind up being world's apart.
justinsayne: "This kinda surprised me as I was one of the many people who thought Thanos was in line as teh next major threat to the Avengers, it certainly seemed that way after the way Avengers ended. That said I can't say I'm disappointed in this news as I think Ultron should be a great choice to keep our heroes busy in the next film."
In a way, he might be. Joss Whedon stated at Comic-Con that Hank Pym, the creator of Ultron in the comics, won't be in the movie. Maybe Whedon intends to tinker with Ultron's origin in which Thanos is the one who created him. In the closing credits, I remember the servant, whoever he was, telling Thanos that to take on the Avengers was to "court death" and Thanos smiled. As they foiled his plans made with Loki, maybe Ultron will be used as some sort of "test" and they're ultimately saving Thanos to be the primary villain for the third film.
Vin Diesel has been teasing a role in a Marvel film for a little while now and it's widely thought that Thanos could be him. Diesel sat down with Entertainment Weekly and said:
"I could tell you that Marvel was excited about bringing a different kind of relationship or love story, I guess kind of inspired from the Dom-Letty relationship (from Fast & The Furious), into the Marvel universe."
Thanos has a hard on for Death. In the Marvel Universe, the concept of Death is a sentient being as old as the universe that can actually assume a physical form, has intelligence, motives, designs, etc. Take the concept in the Final Destination films, jack it up to the millionth degree and put it in the form of a robed female skeleton and that's pretty much Marvel's incarnation of death. In many of the more classic storylines involving Thanos, it's his ultimate goal to win Death's affections.
However, because of Ultron's involvement in the Avengers sequel, some are suggesting that Diesel might play The Vision. Ultron creates Vision in the continuity and he's had this relationship with the Scarlet Witch.
At any rate, I've got confidence that Whedon will be able to put it all together.
"What Do I Know Of Cultured Ways, The Gilt, The Craft And The Lie?
I, Who Was Born In A Naked Land And Bred In The Open Sky.
The Subtle Tongue, The Sophist Guile, They Fail When The Broadswords Sing.
Rush In And Die Dogs - I Was A Man Before I Was King."
Conan Of Cimmeria