Mystery eggs are ridiculously popular- can't even tell you how many hours my son has sat there watching people's hands open "mystery eggs."
MysteriousEgg definitely has potential. Mysterious is not a very easy word to spell for most, I assume.
''mysterious'' word is popular with 210 million results. So, i see potential to this. Because this is generic, i don't think that it is issue pokemon fading. In any case, i would never reg a pure pokemon name, because i hate trends.