I dunno, I tend to think that piling things onto bread should work pretty well, provided it's mixed well enough. The guys at the sandwich shop seem to layer things on in this order:
meat, cheese, lettuce (shredded), tomato (sliced) with mustard and mayo on the supper bun. That may just as well be for standardization as for quality, though. I don't think I've ever eaten a sandwich that felt wrong because the things were in the wrong order. (This HAS happened with burritos, though.)
Another place did a strange thing where they wrapped the meat and cheese slices around the lettuce and tomato, like a wrap but in a sub bun.
Also I asked some of my family for their thoughts on this question and they say to only use ingredients that are good individually (like, that you'd be willing to have a bit of by themselves), to have some balance (balance is practically the whole point of the sandwich, do keep it in mind!), and to NOT USE WHITE BREAD, which does not qualify as food.
As you might imagine, we're all pretty passionate about sandwiches here.
...And while I was away with this post half-written to eat dinner, SIX replies come up, almost all about the stupid hotdog question, which isn't even relevant here.
Regardless of a hotdog's classification, one should apply the same sort of thinking: use good things, have some balance. The hotdog may just be a wad of meat in a bun by default so you can have a bit more leeway in terms of disbalance, something like relish or sauerkraut will act nicely.
So; there's your real reply to this very serious and dire question. Shame on all of you who replied with shitposts to this poor wayward soul, merely seeking advice on sandwich enlightenment.