Last night I came up with a (IMO) very solid guess for who the new Megas are for the demo games. I browsed through the discussion in this thread and did not see anyone who had come up with the same thought so I figured I would share it now. If it has already been said and I missed it my bad!
I am feeling very confident that the new demo Megas are Latias and Latios, for 2 main reasons. Firstly the advertisement for the demo says that if you play the game multiple times special things will happen. I think there are several choices you can make in game and those choices result in you getting different Mega stones. The second reason is that Mega Lati@s are the only unreleased Megas that are compatible with X and Y. If the new Mega is not Lati@s then its stone will not be recognized in game and we will have to wait till ORAS before we can transfer it. It explains things perfectly and I would be amazed if I am wrong.
Assuming that I am right then there is one other possible implication. There is a chance that the Demo might be the only way to get Lati@s's mega stone and its not available in ORAS. I say that as their previous signature item (soul dew) has been event only in the past, why not do the same with the Megas? And if Mega Lati@s is available in ORAS then it would make the demo insignificant and fewer people would want to get it. I suspect their plan is to create a buying frenzy so that everyone is desperate to get Mega Lati@s while they can (code numbers are limited, they have already ran out in most of Scandinavia). I hope I am wrong on the second theory, what do the rest of you guys think?