6. Ant-Man
Ant-Man's own film isn't due for release until the end of 2015, so this might be jumping the gun a little, yet despite its unlikelihood, an Ant-Man cameo would be a great way to nod towards not only The Avengers 2 but even Phase 3, which Edgar Wright's hugely-anticipated Ant-Man film will appear in. If Shane Black has somehow managed to shoot this scene already and keep it under wraps, then it'd be hugely impressive to have the actor cast as Ant-Man revealed to us at the end of the film without prior warning; it would (hopefully) keep us buzzed as we leave the cinema. Given that Iron Man 3 reportedly features a version of Radioactive Man (planned to be played by Andy Lau before he dropped out), it would make sense for Dr. Hank Pym aka Ant-Man to make a cameo considering the film is clearly trying to tie all of the characters together into this huge Universe.