As I recall, the team has the Misty Mountains planned as the last faction to be released, so unless there's been a massive programming/modding breakthrough that they've been keeping under wraps, or unless Imladris is giving them more trouble than MM, we could see Rivendell as the next faction released. Just my guess.
The Edain Team has not announced any future plan yet, after the release of Angmar; that is, we don't officially know anything of the next faction they wish to release.
I think you might be referring to the common opinion within the Community (that I too embrace) that the Misty Mountains will be the last faction to be released, given its current development progress (this is what has been officially stated), its complexity and its own characteristic status of, arguably, the most Hobbit-based and innovative faction among the other ones (thus, along with the end of the Hobbit trilogy, it would symbolically close the cycle of the new Edain Mod 4.0 in case it came out last).
They are still speculations, though very reasonable :)
As for the celebration, congratulations to everyone who has been involved in this extraordinary journey (still continuing), from the earliest times to the latest ones as well.
I personally believe that the fact that both the Mod and the Community (above all, the establishment of the English Forum on MU) went really and officially international contributed greatly to this well deserved success ;)
Happy Birthday Edain Mod 4.0! 8-)