Not to ruin any speculation on WHO the "HulkKing" is, I know they are talking possibly this person, that person. I believe the Hulk King is The Leader, also a Hulk, and big time Supervillain of the Incredible Hulk. Mr. Blue, the scientist in the Incredible Hulk hits his head on radioactive material due to the Hulk hulking out. Once a friend of Banner, now an enemy. The Leader is also slated to be the nemesis in Captain America 4, so it makes sense that they would start bringing him out now, as they did Thanos, years prior to the movie he would be in, himself.