I was hoping for Ultron over Thanos. I guess they'll use Stark as Ultron's creator (directly or indirectly), unless Whedon surprises us with Pym prior to Ant-man.
It's has to involve J.A.R.V.I.S somehow. The thing's practically sentient now.
-He's shown fear when Stark landed in Tennesse
-He's shown worry when Stark was carrying that nuke
All it needs is a body and some bad programming.