For your consideration, I have constructed what I am calling a “modal argument for Molinism,” relying on free will. These concepts have cropped up and even been developed other places, so it’s not particularly original. However, it might be helpful in framing the debate between Molinists and non-Molinists.
1. If libertarian free will (LFW) is possible, then there are truths about how possible libertarian free agents use their LFW.
2. LFW is possible.
3. Therefore, there are truths about how possible libertarian free agents use their LFW.
4. It is possible that all counterfactuals of creaturely freedom (CCFs) have truth-values.
5. If (2) & (4), then there are at least some libertarianly true CCFs concerning creaturely essences who never exist.
6. Therefore, there are at least some libertarianly true CCFs concerning creaturely essences who never exist.
7. If (6) and OMNI, then a middle knowledge account of God’s omniscience is correct.
9. Therefore, a middle knowledge account of God’s omniscience is correct.
Let “OMNI” stand for the thesis that for any proposition p, God knows p and does not believe not-p. (1) seems to be definitional. After all, it means that there is at least one possible world where libertarianly free actions are exercised. (2) may need some defense; admittedly, this won’t do a bit of good against the objector who thinks that, as a matter of fact, LFW is incoherent. But it certainly seems to us to be the case that we possibly have LFW, and, typically, we don’t modally perceive something so strongly that isn’t even possible. That is to say, our modal intuitions, on the face of it, count for at least something, and in the absence of other, stronger intuitions or evidence, we are justified in holding it. (3) is an entailed conclusion.
Similarly to (2), (4) can be denied by those who think it’s impossible that there are any true CCFs describing any libertarianly free agents. This might be open theists, who claim that such truths would render LFW actions impossible, or those who think there are no CCFs at all. However, people should accept (4) independently of (2). That is to say, one can accept (4) whether or not he believes in LFW, or even its possibility. (5) is an analytical truth: if it’s possible that all CCFs have truth-values, and LFW is possibly true, then there is a possible world such that creatures with LFW exist and would use it to perform particular actions and make particular choices. Thus, there are truths about how possible creaturely essences would act with LFW who never exist. (6) is thus an entailed conclusion. (7) is also analytic: granting OMNI, and (6), it just seems almost definitional. God would know about these libertarianly free CCFs in possible worlds, and they are not true due to his decree. Thus, middle knowledge is correct!
The upshot of this entire argument is not to convince the anti-Molinist to become a Molinist. Rather, it’s to frame the discussion in terms of the following: if libertarian freedom and CCFs are even possible, then they are known to God prior to the creative decree—even if, in fact, God does not actualize a world with libertarianly free creatures—or even if God does not actualize a world at all!
 I do recognize classical Thomists would be scandalized at this, and thus they have an out by denying (7). However, I find LFW on Thomism to be sketchy, at best. But to each his own.
 Technically, this is not possible. However, all I mean by this is that God doesn’t create a world; the world he actualizes is just a world in which the Trinity exists alone.