Part of what makes this question so tricky is that Edgar Allan Poe is such an iconic writer that many of his stories become predictable not through any fault of the writer but, rather, due to these stories' exposure in pop culture and their influence on other works. This is especially the case with his more famous stories (of which "The Masque of the Red Death" would apply).
In general terms, I don't think it would be entirely unfounded to label the story's ending as, at least on a general level, predictable. After all, consider the story's underlying story structure, by which a group of nobles and aristocrats lock themselves in a princely estate to escape the ravages of a plague, spending their time in hedonistic festivities. As far as a deeply unsatisfying ending goes, imagine if the story were to end with the Red Death running its course, the partying coming to an end, and all of the guests going their separate ways. Such a resolution just doesn't work in dramatic terms.
With that in mind, the story's setup and structure practically require that it take a dark turn, with the grim reality of this plague intruding on this celebration. One can easily imagine, for example, a twist ending by which one of the celebrants had gotten infected shortly before arriving at the palace, only to start showing signs after he'd already gotten in contact with the other guests.
In a different sense, however, what gives the story's resolution a far more unpredictable quality is the appearance of the specter, a personification of the Red Death itself, which brings with it a supernatural dimension that defies any rational explanation. That the Red Death, in some fashion, should intrude upon the partying is close to a necessity given the story's structure; that it should somehow arrive personally, as a gothic, deathlike figure, is a twist that I think is far more difficult to see coming without possessing previous knowledge of the story's plot.