*когда-нибудь. Когда-то is used when you speak about past time.
*мне интересны восточные страны
*Russia cannot be referred to as восточные страны. Japan and China may be called восточные страны though no one says that. Страны Восточной Европы or восточно-европейские страны, but it depends on where the speaker is as he says that. If you are in Poland that makes no sense.