It doesn't feel as grand as its depicted, and the matches end so fast that you realy don't get to level up much.
And the lack of a campain and persistant upgrades (level ups/ purchases/ etc) makes the game seem kind of mindless to me...
What would have been cool is a campain (doesn't have to be all that long) for each demigod; a means to show how they proved to be worthy of this competition that they're in.

Hmmm...the girl appears to have the body of a teenager, my guess is 15-16.
(...If you look close she twitches and tilts her head to the right just before the 'sand castle' city sprouts up...)
I'm guesssing that the game takes place about ten years after the first; regardless of the ending you play out the world would have found out about Rapture: either from a submarine crew spotting debri and getting attacked or a group of survivors who had escaped with a story to tell of their adventures.
So someone would have thought "wow! those Plasmids sound realy cool; I want them". I think the game is centered around the fallout of an attempt to plunder and make use of the genetics technology in rapture.