What I think should happen to the upcoming movies:
Iron Man 3: The villain in this movie should be either Mandarin or Titanium Man (Boris Bullski). A plotline for Mandarin could be a discovery of a Makluan Ring, where Iron Man and War Machine (possibly) are alerted by an unknown presence, as well as looking for the rings, which could provide a plot for an Iron Man 4 or a 5. With Titanium Man, it could be that Russia wants to prove their superiority by inducting Boris Bullski to wear the Titanium Man armor. Possible characters in this could be Bruce Banner, due to him being with Tony Stark in the end of the Avengers movie. War Machine is a possible as well.
Captain America 2: Possible villains in this movie could be Crossbones, Baron Zemo or the daughter of the Red Skull, Sinthea Schmidt. A plot for a movie with both Crossbones and Sinthea Schmidt, is that Sinthea Schmidt and Crossbones are looking for the Red Skull after the events of Captain America: The First Avenger. Sinthea Schmidt could also have recovered Bucky Barnes, which could lead to the events of a possible Winter Soldier movie or a third Captain America movie. For Baron Zemo, it could be either Heinrich Zemo or Helmut Zemo, where HYDRA returns and Captain America is tasked with the destroying of the organization. This could be linked with a possible Nick Fury movie or a S.H.I.E.L.D movie. Possible additional characters could be Sharon Carter, daughter of Peggy Carter and a recruit in SHIELD.
Thor 2: Possible villains in this movie could be the Enchantress and her Executioner. A plot would be that the Enchantress and the Executioner were able to return Loki from his imprisonment and unleashed the Executioner on Earth, while Loki and Enchantress unleash the Frost Giants towards Asgard. Possible characters could be Thunderstrike or Beta Ray Bill.
Ant-Man: Henry Pym will be involved, possibly with the Wasp. Villains could be Egghead (Elihas Starr), the Black Knight (Nathan Garrett) or Ultron. They could be approached by Captain America or Black Widow, to join the Avengers initiative.
The Incredible Hulk 2: Possible villains could be Tyrannus or the Abomination again, as well as the Leader. A plot could be that Tyrannus kidnapped Betty Banner and Hulk goes underground towards him. The Abomination could be involved as well as the Leader, who masterminds the attacks on Earth. Iron Man could be involved, helping Hulk against the threat.
Black Widow: Black Widow could still be involved with SHIELD, being dispatched in Russia to deal with a threat with the KGB and Hawkeye could be dispatched to help. The Tinkerer could be in Russia, selling weapons to Russian terrorists. Yelena Belova, the second Black Widow could be involved as well.
Hawkeye: Hawkeye is involved with SHIELD and encounters Trick Shot and Swordsman and fights them with the help of Black Widow and Maria Hill. Trick Shot and Swordsman could be terrorists working for HYDRA and Hawkeye is reminded of his past.
Avengers 2: Possible villains could be Loki, Abomination, Baron Zemo, Titanium Man, Swordsman and Ultron, forming the Masters of Evil. Thanos could be involved in some way, directing the Masters of Evil towards key cities, destroying them. Possible new Avengers members could be Ant-Man, Wasp and War Machine.