I like the idea of a "non-human" (like the Pepsi "Play for a Billion" Monkey) being the boss, but I think that the firings are only quasi-random, so there has to be some other factor involved.
I like the idea of a Rupert or Clinton as the boss, but it doesn't fit with the overall image of the show. Who would ever complain about working for one of those two? The show is set up for a real shock ending...
I like the idea of "The Man" being female (i.e. Amorosa), an homage to the first/second season of Alias... But there aren't many "shocking" females that I can think of.
In fact, there's only one human I can think of who would qualify as sufficiently famous to be instantly recognizable but winning a job to work for would be truly embarassing and shocking:
That's my guess.