I too want to see the original dragon that the trio was born from, I was sure that it'd be part of Black and White 2, but nope, we only got to see 2/3 of it. I don't think we'll get to see it now that Gen 5 is over, I really doubt that they'd overshadow the central plotpoint of a future generation by making the Gen 5 story come in and steal the show again.
The only reason Mewtwo might do this is to feed off the nostalgia from people who watched the first movie when they were kids, and the fact that Generation 1 is the fan-favorite game, mostly due to it having such a nostalgic impact on people as this was when they began to play, even if the game wasn't that good, especially not compared to today's standards. It shares this with Vanilla-WotLK WoW and Super Smash Bros. Melee.