You didn't see his 170 knock against Auckland, showed awesome discipline there, would've probably stuck out the 200 if he had more runs to chase. McCullum is the sort of player who can out hit anyone on his day, but those days are certainly not guaranteed. He is probably the most likely candidate for an ODI 200 out of the current batsman, but I really can't see it happening against one of the worlds better bowling attacks (SA, Aus, SRL, Ind, full strength Pak and Eng), yet eventually he's going to just slog his way to a hundred with 40 overs left in the match and he will have ample opportunity. Whether he has the nerve to carry on is the real question.
As for his S/R in ODI's this is due to his position down the order, if you look at his S/R at the top of the order only than it is far higher, in Tests he still only making his mark.
Personally I reckon it's not ridiculous to say he could be the next Gilchrist or Tendulkar (in ODI's, never Tendulkar in Tests) but it would be overrating him to say that he is the next Gilchrist or Tendulkar. He has potential, but a long path to walk until he can be added to the names of the greatest attacking players ever.