One of the bad parts about fans predicting rosters is most of them fail so separate what they want from reality.
True, mewtwo, lucario, and charizard would indeed providence more hype, but if they NEVER had a mega evolution, most fans would not even consider their chances of getting in. Jigglypuff on the other hand has been in from the start meaning he is almost certain to get in. And if you want to add the "new" additions for X and Y, he also gets fairy type, which increases his almost certain chances. Now the only other factor is when he will be revealed, will it be this week, or an unlock able character when the game is released?
Considering all of that, if a pokemon is revealed it will most likely be Jigglypuff.