To date, at least in my opinion, Thanos' appearance in The Avengers post-credit tease has yet to be topped.
Damion Poitier portrayed the Mad Titan for The Avengers, and with a simple smile revealed more about Thanos than James Gunn did in Guardians of the Galaxy and Whedon did with The Avengers 2 (CGI with Brolin voice).
Now the actor that portrayed Thanos in The Avengers movie, Damion Poitier, has been spotted on the set of Captain America: Civil War in an unknown role (note: Poitier isn't playing Thanos anymore).
Poitier is also known for doing stunt work - he was a stuntman on Thor 2 - so he could be back doing stunts for Captain America: Civil War as well.
Poitier did previously state he was in talks for a legit Marvel role, so this could be it.