I think it humorous that other studios, like Sony and Fox have rights over MARVEL'S PROPERTY! Still, those points are 100% dead on. However, I wouldn't be surprised to see cameos, at the very least, by characters like Spider-Man and Wolverine.
We KNOW that there is going to be an Avengers 2. If you stuck around for the FIRST (yes, there were TWO) stinger during the credits, you know who the villain will likely be. So who will be joining the movie? Rumors are already swirling, perhaps completely fan driven, of Wasp and Ant-Man/Giant-Man. I've also already heard of the possibility of The Scarlet Witch, Quicksilver, and less likely, Dr. Strange. They could also go into Wakanda and encounter The Black Panther. Even more so, if they go up against the villain seen in that stinger, then the Fantastic Four and Silver Surfer could make an appearance. I know, WAY too early to start speculating. We can start when Iron Man 3, Thor 2, Captain America 2, and Hulk 2 (I refuse to count that Ang Lee quagmire), which all have tentative release dates.
Now, if you haven't seen it yet, or did see it but didn't stay long enough, there ARE 2 stingers, and the last one is pretty good, well worth sitting all the way through the credits. If you have seen the movie but didn't stay all the way to the very end and want to know what it was about, PM me and I'll tell you.