Season 2 finale shocker · 8:47pm Mar 9th, 2012
The Following post is merely speculation, it is not intended to be taken as a reveal.
Reference :http://www.equestriadaily.com/2012/03/tv-guide-released-twilight-sparkle-and.html#more
I've come up with a list of possibilities for what the Shocker could be,
Princess Cadence...
* Is disguising her alicorn nature. [A rather tame guess. Possible that she feels that showing off that she IS an alicorn might entice others to treat her like they do Celestia and Luna, and she doesn't want that.]
* Is disguising herself as an alicorn [The Inverse of the above, a pony who wants to be treated like royalty.]
*has an embarrassing hobby or lifestyle. [Gambling, larping, enjoying TV or radio dramas, anything at all that could be 'improper' in polite society]
*is actually Prince Cadence. [This would actually be a meta lesson for the viewers, not to judge someone based on who they love]
*is part of a consiracy against the Princesses, or Equestria. [Secret agent Cadence]
*is Trixie in disguise. [Marrying Twilights brother to get back at her for the humiliation she suffered in Boast Busters]
*is Discord in disguise. [Do I even need to explain this one?]
*is a parasitic bride. [She could be marrying Shining Armor with the intent of gaining his wealth. Alternatively, she could have the intention of draining his life, and using it to sustain her own unnatural existence.]
I don't believe there's any likelihood of Candance being an antagonist or a bad pony at all. She was clearly ordered by the toy department, who demand a "pink princess pony" to go on store shelves, not to mention the whole wedding playset. You don't do that with a negative character. I think we can scratch several of your scenarios off the list on that basis.
Looks like it was a combo. numbers 5 and 8. As well as the Evil twin scenario.