To exist = to have an actual value
Not existing = to have No actual value, no value
To a modal realist, even a possible world that isn’t the actual one has the potential to exist in the way you’re defining existence. But they’d say that everything exists, and that what’s actual is what’s stipulated from observation, and what isn’t is what isn’t. But mere possibilities still exist. I think we need to come up w/ existential categories for different types of existence. I bet someone’s done it before.