Presumably, when G-d told Adam and Eve not to partake of the fruit of the Tree of Knowledge of Good and Evil, He meant it. So my question is, what did G-d have in mind for them had they not sinned, which I assume was supposed to happen?
There is a certain strain of commentaries that explore the idea that G-d foresaw, and therefore intended, that man would sin, and that was part of G-d’s plan for enabling his prime creation to perfect itself through free-will decisions and ultimately attain G-d’s image through the circuitous route of history as we know it.
But if we presume as you posit, namely that G-d intended for them to perfect themselves by overcoming that one, primal directive, the intended scenario would have been as follows:
Though created in G-d’s image possessing the potential for perfection, Adam and Eve still needed to realize that potential through their own effort. This would have two basic results. One, they would be able to completely cleave to G-d, having effected their perfection. Two, they would deserve the reward of that closeness to G-d, having accomplished it themselves.
This means that if they had just observed that initial commandment, they would have perfected themselves, realizing their G-d-like potential, and would have lived eternally with their purely begotten offspring in an eternal Eden in the presence of G-d. In fact, the mystical teachings of Judaism explain that if they had just elevated themselves that fatal first Friday, they would have ushered in their first Shabbat in a state of near perfection, which would then have been completed through the holiness of that day. After having attained that plane of holiness and purity, enabling them to understand G-d on the highest levels, G-d would have then allowed them to partake not only of the Tree of Knowledge, but also of the Tree of Life.
Having failed the test, rather than perfecting themselves, Adam and Eve incorporated within themselves the venomous toxin of rebellion against G-d, which was in turn commuted to their offspring, humanity. This initiated the contingent plan of world history whose more arduous and circuitous path to perfection we’re still negotiating today. And until the Knowledge of G-d is attained through our own self-perfection, the elusive, esoteric elixir of Eternal Life is hidden in Eden.
Ultimately the Truth of G-d, righteousness and morality revealed by the Creator in the Torah to humanity will inspire the Knowledge necessary for mankind’s return to Eden. In fact, Rambam writes that Christianity and Islam, whose truths lie solely in Judaism, play an important role in this spiritual transformation by weaning humanity from idolatry and immorality. Eventually, the unfortunate distortions of these religions will be reconciled with their source, the Torah. And all humanity, Jews as Jews, and non-Jews as Righteous Gentiles, will live the ethical monotheism revealed by G-d in the Torah for all mankind.