He already had several comebacks - so why not get anothe one in Avengers 2.Thanos is also known to be quite well connected: He has fallen for Death – who is – no surprise there – a woman and thus after his latest defeat has last been spotted standing with his mistress.
With his superhuman strenght, endurance and reflexes, supported by energy blasts from both his hands and mind and his Infinity Gauntlet he seems a fitting adversary and threat who could easily raise some havoc in a 21st century Marvel Avengers Movie.
The Infinity Gauntlet also provides him with control over time, space, reality and souls of others. Not as good looking as Loki, but damn effective.
Thanos had his first appearance in an extended storyline of Iron Man #55 (Feb. 1973).