Kretschmann will play Baron Wolfgang von Strucker, a character with a long and, in mighty Marvel tradition, complex history. The character first appeared in 1964 doing battle with Nick Fury and his Howling Commandoes during World War 2. Subsequently he's taken over as the head of terrorist organization Hydra, died and been revived and fought with Captain America, Wolverine and most of the other Marvel heroes at some point over the years.
Along with most of the original cast returning and Joss Whedon back in the director's chair, it's already been stated that James Spader will be taking the role as main villain 'Ultron', a psychotic robot hell bent on destroying the human race. Other new cast members include Elizabeth Olsen as the Scarlet Witch and Kick-Ass star Aaron Taylor Johnson as her brother Quicksilver.
It remains to be seen how big a role von Strucker will play in the sequel but Kretchmann, who has some experience playing Nazis, having done so in Stalingrad, Valkyrie, U-571 and Downfall amongst others, should bring some presence to the role.