Well, it looks like all the rumors regarding who would be working with Megadeth in different capacities has been true! Lamb of God's Chris Adler is definitely drummer on Megadeth's upcoming album, though not joining as a permanent member, and now we've gotten official word that Angra guitarist Kiko Loureiro has joined as a full-time member! According to an announcement posted by Dave Mustaine on the Megadeth website:
I first met Kiko around 8 years ago for a cover shoot for Burrn! magazine. I had no idea who he was, other than the fact he was tremendously talented and that the staff from Burrn! held him in high regard. Since then I've come to see what a guitar virtuoso he is, and I'm deeply encouraged by his depth and talent. Very few Megadeth alumni have had the same feel and ability as Kiko. As Frank Sinatra says, "the best is yet to come!"
No word on if Loureiro is writing with Mustaine or not, though it would make sense given that we're kind of sure that Adler might be.