I think *just* Ultron and an army of robots would get boring real quick to general audiences, though. Robot armies are dull as hell, and so been-there/done-that. Battle droids and New Trilogy all over again.
And I don't think audiences want to see another alien invasion in Avengers 2 --- makes the team look like a one-trick pony.
I think the sequel needs to stay on Earth, against Earthly villains. Ultron, yes, obvious choice; but if it's *just* an army of robots, then fail. Ultron as part of a Masters of Evil team? Now we're getting somewhere.
And I think Kang *could* work great in the sequel. Time travel is not the same thing as an alien invasion; and an army of crosstime mercenaries could actually look pretty cool. I'd love to see the Eternity War on the big screen.