1. Black Panther
With all these rumors going around about the Black Panther appearing in Age of Ultron due to director Joss Whedon filming some scenes in Africa and with Wakanda located somewhere hidden in Africa, Black Panther is at the top of the post credit scene cameo list and Chadwick Boseman has been rumored to portray the character in the movie but not officially confirmed. Same goes for the chracter. All of this will depend on whether Joss Whedon will allow it or not.
2. Mandarin (real Mandarin of the Ten Rings)
Leader of the Ten Rings organization and Tony Stark's greatest enemy, the Mandarin has not specifically appeared yet in the Marvel Cinematic Universe and the one played by Ben Kingsley in Iron Man 3 was a decoy and an experimental version of the villain and the One Shot that Kingsley starred in confirmed that the real Mandarin of the Ten Rings is still out there and because Marvel got started on the script to the Avengers sequel after the premier of Iron Man 3, there is a slight possibility that the Mandarin (the real Mandarin of the Ten Rings who hasn't been seen yet) may appear in one of the two post screen credit scenes or after the Avengers defeat Ultron where Scarlet Witch and Quicksilver would inform Baron Strucker about the outcome of the final battle if they survive against Ultron and leave Strucker ttowait on further orders, the Mandarin would show up and confront Strucker to confirm that what they told him about was true and after more brief discussion on other certain topics, Mandarin will threaten Strucker to stay out of his way and leaves which would hint a possible plot for Iron Man 4 but certainly not Avengers 3. Perhaps a war between the Ten Rings and HYDRA. Surely it doesn't seem likely but having the real Mandarin of the Ten Rings cameo in the movie should help prove even further that the Mandarin will appear as the real power ring blaster for Iron Man 4 and since Mandarin has a distaste for technology and it was because of technology Zola was able to make Hydra compromise SHIELD, and if the Mandarin has connections with any world officials who opposes HYDRA, chances are whether HYDRA existance in the 21st century is the reason why Mandarin formed his organization in attempts to ensure they don't take control of the world remains unknown and we could be looking at a gang war between the two terrorist cells in the future.
3. Wasp
While not yet officially announced, Wasp, as Janet Pym may appear in one of the Avengers sequel's post screen credit scenes but that will be if Evangeline Lilly is confirmed as Janet for Ant Man
4. Ant Man
Possibly Hank Pym, Douglas would cameo in the film which then follows on with Ant Man
5. Dr. Strange
Recently mentioned by HYDRA mole Jasper Sitwell in Captain America the Winter Soldier, there is a small chance that Dr. Strange may get aquainted with one of the Avengers, maybe Scarlet Witch
6. She Hulk
As Bruce Banner's cousin, Jennifer Walter's cameo appearance in the film would confirm her appearance for the next MCU Hulk movie
or none of the above depending on what Marvel or Joss Whedon has in store for the film
To find out be sure to resume on keeping your eyes open for any updates on the film's post screen credits