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.