Pokemon releases have been more or less annual since they first came out, with only 3 exceptions in the 15 years they've been making games. It's likely that by the end of 2014 we'll have some new form of main series game, whether it's Pokemon Z, Hoenn remakes, or a new generation. I personally find remakes and a third version equally plausible, since in the past they've done both a third version followed by a remake(fire red and then emerald), and also a remake followed by a third version (platinum and then heart gold).
I had the main point that because of this, we'll probably see the other two event legendaries by the time the game releases in 2014. However I've now decided that they could easily hold off on revealing the other two until after the third version comes out, which could be as late as 2015 if they do remakes this year! I don't think we can read to much into this early release of Diancie since they have a lot of potential material for this generation and they could stretch things out a lot, or not.