Can ijtihad be a source of religious ruling?

To me, "ijtihad" is an opinion of a person in matters of religion when, no direct guidance is available from Quran and Sunnah of our prophet, Muhammad (PBUH), about that topic. This person is usually a religious scholar. My point is that, Islam is religion from God, which was conveyed, firstly, by Prophet Muhammad (PBUH), and then through generations of muslims, to us. Since, the religion is an eternal guidance from God, no one has the right to add, remove or change anything in it. If this is so, then, any opinion provided by him should be taken as a opinion and not an order from God. Thus "ijtihad" can not be a valid source of religion. Is this reasoning correct? or is there any proof in Quran and Sunnah that "ijtihad" can be add, remove or change anything in the religion of God?