There's no way to be sure yet. But it could be other types of hunters we haven't seen before. Maybe hunters from a far - off land with unique nen abilities.
The old and new story of Hunter x Hunter do match to a large extent. The lore and the power system are maintained throughout. The old story introduced us to the unique and complex power system of Nen, which is still very much a crucial part of the new story. Also, the overarching goal of exploring the mysterious world of Hunters and the secrets it holds is continuous. Even though new villains and challenges emerge in the new story, they fit within the framework established in the old parts.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.
The Hunter x Hunter manga has indeed concluded. However, the ending might have left some fans wanting more or with mixed feelings depending on their expectations.