The Official Declarations of the church changed practices, and not doctrine. Although larger in scope, they are similar to changing the age of missionaries. I don't see a practice or a doctrine that needs to change. But I will try to think of some options.
The first official declaration was about plural marriage. No doctrinal change was made with the declaration, but rather it was an affirmation that the church was complying with the law. The Republican Party was founded, "prohibit in the Territories those twin relics of barbarism — Polygamy, and Slavery." The prophet saw what would happen to the church if they fought this law and shift in culture. Plural marriage wasn't required so it was ended in favor of benefits to the members of continued use of the temple for other purposes.
Second Official Declaration gave all worthy the priesthood. This change ended the ban on blacks. Interesting to me is that no justification for the ban or the lifting of the ban was given. President Young started the ban and promised it would be removed. It was. No doctrinal change was made, only a practice of the church was altered.
So the first was a halt to plural marriage in order to comply with the law. The second ended a practice that had no apparent purpose, but may have been tradition only.
I don't see it going so easy for number 3. What needs to change? What doctrine is wrong? What principle is the church getting wrong? What scripture is read wrong? What part of The Family: A Proclamation is in error? The importance of family in God's plan is pretty clear. This isn't like complying with a law for cultural acceptance or ending a practice that no understood or could justify. This is pretty basic. Homosexuality is sin and only men and women can marry.
One blogger wrote that she found peace knowing that her son would just continue to be gay in heaven. Would that change in a declaration make people happy? I don't think it will happen. We all have to give up all of our sins to go to Heaven. Jesus will take them away. He will heal all. We don't get to keep them as souvenirs. Being gay is no exception to that rule.
Do people expect the Church to recognize same sex marriage? That isn't possible. Marriage is between a man and women. Should gender be ignored? Just kinda look the other way and let two men get married, without actually saying what sex they are? The proclamation on the family declares that gender is essential and eternal. We have to rely on that because no other prophet was asked this question. Apparently same sex marriage was never tried in Nephi or Isaiah's day. This appears to be a new invention. Modern prophets have declared that it is false and not compatible with the plan of God.
Could all that be ignored and allow gays to marry now and wait until the next life to sort things out? This isn't like waiting for the blacks to eventually get the priesthood as was promised? What prophet suggested that two men could be married? Who said eventually people could be sealed to whoever they wanted, regardless of gender?
Marriage is the union of complementary sexes, anything else is just pretend. Same sex marriage cannot be sealed by the power of the priesthood and the Holy Ghost and therefore all such unions will end in the grave. Two men cannot be joined in the next life. Only a married couple can enter the new and everlasting covenant and obtain the highest level of the Celestial Kingdom. That is the plan. Unless Declaration 3 will say that Joseph Smith got that wrong. Men in a legal union here will be single in heaven. Without accepting the true definition of marriage and participating in it, they can have no increase. Those that practice homosexuality are sinning in this life and will be limited in the next.
Do gay Mormons expect the plan to change, or be clarified in some way, and that somehow they will be accepted as a pair capable of producing life in the next world, even though it appears to be in violation of God's plan and the rules of nature He established? Will Official Declaration 3 claim that they can have successful and lasting relationship outside the plan of happiness established by their Creator?
My question is just what doctrine is expected to change? The family proclamation provides a great summary of Hesse issues. I don't understand what part gay Mormons expect will change. Gay Mormons or those that sympathize with them, I should say. Speaking of which, I believe we must love all. All are welcome in the church as we share this mortal experience and learn to avoid the snares of the Adversary. I hope people can find clarity on this issue and trust that our Heavenly Father wants what is best for His children. Jesus loved and made God's plan real. Jesus died for all pains and sins. He can heal all hurts. I don't think there will a third Declaration on this issue. The correction isn't required for the church but rather, must be made in individuals and in our society that perpetuates this evil.