[MoonlightTabby]

What I had immediately thought was that there would be a randomly generated pet inside, not something too special though
Like, you wouldn't see a pegasus in there or something, probably just a randomly generated fairly cheap pet 

Though, I do like what Wenom said, perhaps there could be baby petz that you could only find in the mystery eggs and nowhere else?
Or maybe you could get pet eggs that would eventually become full grown petz :o
I know that's a bit of a stretch but I kind of like the idea c;