It's rather low priority, but it'd be nice to have built-in set functionality (here meaning an unordered data structure where no duplicate values can exist). Associative arrays can already fill the role to some extent if you just opt not to use the values at all, but that's a waste of storage.

Syntax could even be similar, using "void" in place of a value type:

---
void main () {
    void[string] set;

    set.add("hello");
    assert("hello" in set);

    set.add("hello");
    set.remove("hello");
    assert(!("hello" in set));
}
---

If creating a new property ("add") is out of the question, even something like...
    set["hello"] = true;
could work.