The proposed unless is nice, but I’m pretty sure the price of an extra keyword is too much for most.
If you can somehow make this work with existing keywords it may fly. But then, what’s wrong with using negation.
If there is any control structure I miss, it is the repeat/until loop. You know, the loop that gets executed at least once. Like do/while in C/C++, but with inverted condition. If you need to execute a loop at least once and all you have is a while loop, it feels unnecessarily awkward to implement.