I think the perfect sandwich is the exact sandwich you need at any given moment!

One important aspect of a good sandwich, to me, that gets overlooked in sandwich discourse* is textural contrast, for example, if the fillings are all soft, it's good to have a crunchier bread (baguette sandwiches, grilled cheeses), but if the bread itself is soft it is good to have at least one element that's more crunchy inside (lettuce on a burger, katsu sando).

*If only Ian Bogost were on here