I'd say ingredient neutral for sure.
Structure neutral too, but only for the sub type. (Not taco type. I don't consider tacos as sandwiches).
I grew up with sandwiches that were made of baguettes, and either fully open or not, depending on preferences or at random, (or because you tried to make it not fully open but the knife cut too much
) so I can't really consider that as a relevant feature of what makes a sandwich.