If we're not actually talking about things, then it's not philosophy, it's just word games.
If something doesn't exist, then nothing can be truly predicated of it. The problem with the following syllogism:
All rational beings are persons.
All Martians are rational.
Therefore, some Martians are persons.
is not that I have assumed existential import, thus committing a formal fallacy. The problem is that the minor premise is false. It is false because Martians do not exist.* Therefore, nothing can truly be predicated of them.
It is not a good idea to mathematize and abstract logic to the point where we forget the primacy of being
*If we ever discover that there are Martians, I will obviously have to change my example.