Yeah, rumors are running rampant at the moment. Jarvis will become Ultron, whose new body will be made out of vibranium (can't use adamantium), and this means Wakanda will appear in the movie, since that's where all the vibranium comes from. T'challa will have a cameo, maybe his dad, too. Paul Bettany, who voices Jarvis will do the same for Ultron but will also appear in person, possibly as the Vision. The synthetic man (aka Human Torch) we saw for a few seconds at the expo near the beginning of CA:TFA will also figure in the story. Ultron will assemble a version of the Masters of Evil (as he did in the comics) and so we'll see various villains from earlier Marvel movies. Wanda and Pietro will be in this group but will change sides and join the Avengers.
At least, that's what I heard...