I can guarantee you that you have never - even in your imagination - visited a world where there are square circles or married bachelors. While one could conceive of a world in which dragons existed, since there is nothing in our world indicating that these creatures are logically impossible, we cannot imagine a world where square circles exist. Each, by definition, excludes the other. Philosophers love thinking about impossible worlds, though they can never enter them. This is because impossible worlds are an extremely useful tool we can use to test out the viability of an idea. Running an idea through this test doesn't prove the idea is true, only that it could be true. However, if an idea fails this test, we can remove it from the realm of possibilities and learn important information from it. So let's explore some worlds together by addressing a question I had recently: Is there a possible world where fallen humanity exists, but Jesus does not die for them?
We'll start with the easiest implication first. If you say that there are no possible worlds in which humanity fell that Jesus didn't die, you are undercutting notions of libertarian free will. I am fine with that because I think libertarian free will is a vacuous and incoherent idea. But a lot of people like it. If you're going to say that any time fallen humanity exists, Jesus dies, then Jesus's death seems less like a libertarian choice and more like an inevitable choice Jesus had to make (since in no world could he have ever chosen otherwise). But inevitability doesn't make the choice an arbitrary one. It would make Christ's choice one grounded in his immutable character, which is why the choice is immutable. Since God is love and his definition of love includes enemy love, in every world where enemies exist, so does his love for them.
That brings us to the second implication. If one believes that a possible world exists where humanity falls, but God does not call Christ to die, then loving enemies is not an immutable, core component of what love is. Instead, enemy love is an add-on. It's something God chose to do in our world, not because he is love (since enemy love is not immutable, and therefore not a component of immutable love), but simply because he chose to. Given this answer, while one may be able to defend God's libertarian free will more easily, they would lose the concept of love as the Bible seems to portray it. Such a view would also reduce the value of God's love towards us in our world, since his choice to love us would have been arbitrary rather than grounded in God's nature (it could not have been grounded in God's nature since some worlds exist where God chose not to love us, and God can't defy his own nature). Finally, when God calls his followers to love his enemies, he is no longer calling them to be like God (since God is not immutably an enemy lover, but only loves enemies at particular times and in particular worlds), but rather calling them to make an amoral decision that he himself made in the real world, but not all worlds. While it is God's prerogative to issue commands to us as he sees fit, knowing that God is not defined by enemy love makes loving our enemies seem less weighty. It is simply an area of obedience to God, not an area of conformity with God. Whereas being generous to the poor, refusing to lie, and refusing to objectify others in lust/adultery and hatred/murder carry the weight of adhering to the nature of an immutable God - if God is not immutably enemy love, his command feels much less weighty.
In summary, if Jesus doesn't die for fallen humanity in every possible world, then the immutable part of God's love looks a bit different than most of us intuitively know it to be, and what the Bible seems to indicate. This is where the conversation generally gets hung-up, as many don't like the idea of Jesus "having to" (even if it's based on his immutable character) choose sacrificial death for his enemies People argue that God would be perfectly just to send us to hell without a second chance, therefore, a possible world exists where Jesus chooses not to die. But these individuals are conflating "justice" and "righteousness."
In his work entitled "On Free Choice of the Will," Augustine puts forth the argument that while individuals can kill an aggressor justly in self-defense (and should be allowed to by the state), he cannot avoid seeing how they can avoid condemnation by God's higher law. The church, by and large, recognized this concept for a millenium, as once soldiering became accepted practice, soldiers were often required to pay penance for the lives they took - and priests were not allowed to shed blood at all (even Aquinas agreed with this into the late 13th century). Justice (an eye for an eye) was different than righteousness (paying the price yourself and laying your life down for your enemy). This, some argue, is one of the major distinctions between the Old and New Testaments. In the Old, God often described that which was just. Whereas Ancient Near East laws often allowed for retribution beyond equality, the biblical law drew a line in the sand which prevented more harm being done in retaliation. But just as divorce was given because of the hardness of humanity's heart, so also were these just laws which failed to fully reveal who God is. When the full image and glory of God came, however, he illuminated the fact that God was not content with justice, for justice alone is not righteousness. Lust and hatred were added to the list of evils, as simply not enacting evil was not righteous, though it may have been just, since no harm was done. And love, even for non-neighbors and for enemies, was added to the list of what was required, even though such things went beyond justice. In fact, Paul tells us that without love, anything is useless, and John tells us that if we don't have love then we don't know God in his fullness.
So is God just? Yes. But he is so much more than simple justice. A world where God is justice, but not enemy love, is a world in which God is not God, for there he would not be righteous. .
[See this source for a good discussion of righteousness vs. justice]