The casting call clearly indicates a distinction between at least two "races", one around 5 feet tall, and the other around 6 feet tall or taller.
If the first season focuses largely on Númenor, it would make sense to focus on Men and perhaps Drúedain, who apparently lived in Númenor during the first part of the Second Age, but began leaving during the reign of Tar-Aldarion, and were all gone before the Downfall. Apart from the fact that they existed in Númenor, they're pretty much a blank slate.
If the first season spends time in Middle-earth, there are two regions inhabited by Elves that are likely to be a focus. One is Lindon and the other is Eregion. There were probably no Dwarves or Men in Lindon, but there were Dwarf settlements on the east face of the Blue Mountains, just beyond Lindon. Much more likely, however, during the Second Age the Elven realm of Eregion, where nineteen of the twenty Great Rings were forged, was just next door to the great Dwarvish realm of Khazad-dûm (later called Moria), which reached the height of its glory during this time. This was also a time of unusual friendship between the Elves (of Eregion) and the Dwarves (of Khazad-dûm). To me it would make no sense to exclude the Dwarves if the first season focuses on the Elves of Eregion during the Second Age.
A third possibility may be Orcs. I'm sure they'll be (among) the baddies. It might be interesting to focus on them and their culture, such as it is. Clearly they carried on after Melkor's defeat without starving or killing each other off, so they had to have some sort of social structure. Were they just minions waiting for Sauron to come along, or did Sauron have to persuade them to follow him?
As for Halflings, the prevailing opinion is that they didn't do anything important during the Second Age, and they didn't live west of the Misty Mountains until the Third Age anyway, so there's no reason for them to appear in the series. I generally agree. However, I'm not entirely certain that Amazon's writing team will be able to resist including Halflings in the series. One possibility would be that the Halflings are located around Dorwinion or Rhûn during this time, so whatever they do is unknown to the Elves and Men in the west. However, whatever they do can't be too notable, or it would come to the attention of Sauron, who apparently paid them no notice at all until he captured Gollum.
My money is on Elves and Dwarves.