2. Baron Wolfgang Von Strucker Will Be A Big Villain, And Loki Might Return
Not only does the mid-credits sequence in The Winter Soldier set up the appearance of Scarlet Witch and Quicksilver, there's also the tantalising revelation that Strucker has got his hands on Loki's sceptre. Knowing that the adopted son of Odin is still alive, it's probably a reasonable guess that he will be coming back to get it at some point. But more importantly, Strucker is a formidable foe, even without the ability to control others, and he's likely to either be the big bad of Captain America 3, or it's going to be revealed that he is the big bad working on behalf of a hidden even bigger bad.
So What Next? Strucker is apparently set for an appearance in Avengers: Age Of Ultron, but given that Ultron is the big bad, it's hard to see him playing a role in the main narrative. So, this could be heading in one of two directions: either Von Strucker and the twins will be given a supporting sub-plot, or at the very least another post- or mid-credits nod that sets up Phase 3, where Strucker will be one of the main antagonists, alongside, presumably, Thanos, a returning Loki, Scarlet Witch and Quicksilver, (at least initially) and whoever is picked to play Iron Man's villain if he is to get his own movie (he surely HAS to), or Von Strucker is also behind Ultron. It seems reasonable to suggest that Strucker might be the chief antagonist behind Ultron, if you're going purely on the casting news that he, Quicksilver and Scarlet Witch are to be involved in Age Of Ultron, but the fear would be that Ultron will take too much of the centre-stage and the twins will be relegated to glorified henchmen, who are ultimately, inevitably drawn to the good side of the divide. It would be infinitely better if that doesn't happen until Captain American 3, giving the twins more screen time, and Ultron more focus in Age Of Ultron, instead of putting him on the same level as Quicksilver and Scarlet Witch.