I wonder when Genesect will be officially revealed, though? Some rumours say that in August (already), and if it indeed is shown in the trailer already, there's no point in hiding it any longer. But that leaves almost a whole year before the actual movie comes out and they have to use something to promote the movie within that time. So I really wouldn't rule out the possibility of a 6th gen Pokémon appearing (maybe not a legendary one, but a random one). The games wouldn't have to come out until 2014 or even later, remember Munchlax? It appeared in Destiny Deoxys in 2004 but D/P weren't released until 2006. A similar scenario could happen here.
But I don't know, I guess they could put Jirachi or some other older monsters in as well, or then just use Unova's legendaries again. They could emphasize the possible appearance of Team Plasma (and N?), and knowing their history, I wouldn't be surprised if we have Reshiram/Zekrom/Kyurem + company in this movie as well, haha. I bet that the mention of Team Plasma would get people interested, but we'll just have to wait and see what they're up to! Can't wait!