(A) Plato intended to write Hermocrates but never did for some unknown reason.
(B) Plato never intended to write it.
Why might one suggest a third dialogue that one never intends to write? One possibility is precisely to get people thinking about what it would be. Critias, the discourse by an Athenian, is about an Ancient Athens that turned back and defeated a mighty imperial sea power. Hermocrates of Syracuse, while not a strong general, was an accomplished and respected diplomat whose work in building Sicilian alliances enabled the city of Syracuse to turn back and defeat a mighty imperial sea power -- Athens herself. So just having him there in juxtaposition raises the question of whether he would have discussed this if he ever had his own speech, and, more importantly, raises the contrast between Ancient Athens and the imperial democracy of the Peloponnesian War.
On the other hand, it could be that Plato just didn't get to it. Can we give a reasonable guess about what its topic would be? We have a very few scattered indications.
(1) The task Socrates sets his guests is to "present our city prusuing a war that reflects her true character" (Tim. 20b); that is, to present the ideal city of Socrates in action.
(2) The three guests agree that Critias's story of Ancient Athens fits the bill.
(3) Critias's account of the plan doesn't mention Hermocrates at all:
We thought that because Timaeus is our expert in astronomy and has made it his main business to know the nature of the universe, he should speak first, beginning with the origin of the universe, and concluding with the nature of human beings. Then I'll go next, once I'm in possession of Timaeus' account of the origin of human beings and your account of how some of them came to have a superior education. I'll introduce them, as not only Solon's account but also his law would have it, into our coutroom and make them citizens of our ancient city--as really being those Athenians of old whom the report of the sacred records has rescued from obscurity--and from then on I'll speak of them as actual Athenian citizens. (Tim. 27a-b)
(4) Socrates does explicitly say that Hermocrates will speak (Crit. 108a) and Hermocrates and Critias seem to confirm it (Crit 108b-c).
And beyond knowing from Thucydides and Xenophon who Hermocrates is, this is all we have to go on. Any speculation has to work by extrapolation or analogy. Some possibilities.
(a) After Socrates describes the ideal city in the Republic, he goes on to describe how it degenerates. So perhaps Hermocrates would talk about the degeneration of the ideal city. The problem is that we already know that Ancient Athens doesn't degenerate -- it's going to be destroyed in cataclysm, with only remnants surviving.
(b) Hermocrates, for practical purposes, destroyed the Syracusan Expedition by which Athens aimed to extend its naval empire. So perhaps Hermocrates would talk about that as a contrast to Critias's account of Ancient Athens. The major difficulty here is that the opposition is fairly obvious already, and the topic doesn't obviously flow from the task Socrates set his guests.
(c) Perhaps Hermocrates would pick up from the cataclysm, so to speak, and talk about how to build the ideal city from scratch. Plato does something like this in the Laws. But why Hermocrates? We know from Critias that Timaeus has devoted his life to astronomy, and Critias is related to Solon; all we know about Hermocrates is that he was a Sicilian statesman.
Plato -- even his gaps are interesting. But in this case it's anyone's guess, so feel free to indulge in your own non-reading of the Hermocrates.