Besides breaking just about every law in the Kashrut (The dietary laws of Judaism), every good sandwich needs an "X-factor". Consider Primanti Bros, the quintessential staple in Pittsburgh fine dining. Their sandwiches would not be nearly the same without the addition of coleslaw and french-fries, right between the bread and the meat. After I spend a sizable amount of time frying processed pork products, I usually spend even more time scouring my refrigerator and my pantry to find the perfect "X-factor" element. Building a sandwich is both a science and an art. The egg, whether scrambled, fried, or hard-boiled is a solid hamburger or sandwich topper. It is also no surprise that one of the best combinations with the egg are pork products (ham, bacon, etc.). If there are no eggs to be cooked, I usually try to find something that will contrast the salty taste of the pan-fried meat. Ripping off the sweet flavor of coleslaw from Primantis is always a safe option. However if neither of those can be found, adding another meat to the sandwich equation can be beneficial. Some viable options as add-ons would be: pepperoni, salami, corned-beef, roast-beef, turkey, etc.
As far as cheese goes, I'm no expert. Though I'm not much of a cheese guy, I will try anything at least once (maybe twice). The list of cheeses that I like to eat is expanding with each passing day, as my palate seems to be acquiring the taste. In the realm of sandwiches, a sharp cheddar is always a safe selection, especially if toasters or panini presses are involved. An excellent sharp cheddar cheese not only brings a rich flavor, but also a nice stringy consistency and texture. Provolone is another safe bet, when trying to make the decision between cheeses. A good provolone brings a subtle flavor that only adds to the sandwich equation. The consistency of provolone is solid to say the least, and will tend to hold its shape even when toasted. Then there are the processed cheeses like American cheese and some pepper jack, which can be added to just about any sandwich. I consider these kinds of cheeses primary cheeses, because they can usually stand alone within a sandwich. Some of my favorite secondary cheeses are Jarlsberg and Munster to name a few. These kinds of cheeses can be a great addition to a sandwich with any of the primary cheeses to give a boost in flavor.
Vegetables are an important element to any structurally sound sandwich. Vegetables add color, texture, nutrients and vitamins, and can be a great contrast to the flavor of any protein packed sandwich. Though not everyone's favorite food group, not having any vegetables can utterly break a sandwiches' chances at success. Even the simplest sandwiches need to have at least lettuce and tomatoes (the basics). Vegetables can give a sandwich a much needed crunch that you can get from green peppers or lettuce, or the sweetness from tomatoes or Vidalia onions, which can contrast the tender and salty fried meats. Adding a nice selection of vegetables, like roasted red peppers or pickles, can give a sandwich some aesthetic beauty as well, by adding more color. Not to mention that vegetables will help the nutritional value of some of the more unhealthy sandwiches. Yeah, sure it takes a little extra time to cut and slice these elements, but putting in that extra effort can help a sandwich reach its potential.
Last but certainly not least is the sauce or condiment that gets placed on the bread. Sauce is to sandwich, like cherry is to sundae, or like meatball is to spaghetti (cliché). It is a must. I love a good mustard, and there is really one for any occasion. Deli or brown mustard is good on just about any cold cuts sandwich, but I like it especially with ham based sandwiches. Yellow mustard is the good 'ole stand-by choice, the All-American choice, and is compatible with any sandwich creation. Then there is the magnificent Dijon mustard, my favorite being Grey Poupon. Grey Poupon is the ultimate mustard, because it has a peculiar sweet and spicy kind of flavor that can literally make any sandwich seem like it is the god of the sandwiches (Love). My heart skips a beat every time I open a jar. Then there is mayonnaise, not to be confused with Miracle Whip. Damn Miracle Whip to the fiery depths and the high horse it rode in on. Mayonnaise adds just enough flavor and texture to make any good sandwich, a great one. Next is horseradish sauce. I hold horseradish sauce in the same regard as Grey Poupon. It is god like, and is immaculate on any beef based sandwich, and can add a nice nose clearing kick to any sandwich that comes out a bit bland.
Building a great sandwich is both a science and an art. Trial and error is part of the game when creating something spectacular. Don't be afraid to get creative and experiment with new sandwich blueprints. Open your refrigerator and pantry and get to work!