The eternal struggle – The chicken or the egg, time travel or the traveler, Han or Greedo, you or your significant other – but for my purposes I propose this: What comes first, story or system?
Currently, I have an easy job in that I only use one system in my campaigns; D&D gives me
everything that I need to craft my worlds. I have learned the system and mechanics well enough to know how and when to bend the rules, and which ones I can just outright break, without breaking the game entirely. Therefore I just need to match my campaign to elves and trolls and we are good to go. However, what if I were to expand my horizons? What if I wanted to play a futuristic setting where everyone is a bioengineered human fighting against aliens? Next campaign I might just want to play survival-horror where all of my players are delusional mental patients who's sadistic imaginary friends gain physical form. Tomorrow, I'm going to start everyone as a regular joe with a regular job (you know, your average suburbanite slob) when the undead suddenly rise from their graves and all of the characters are locked inside a CVS surrounded by Zeds.
The proposal of these scenes is the dilemma of nearly every other GM out there thinking of a new campaign – Do I write a scenario and then research to see what system would suit my needs? This can be quite the ordeal since there are dozens upon dozens of systems out there, especially once you start delving into the world of self-publishing you can easily reach the triple-digits. And roll playing systems aren't something that you can just glance over and get the gist of it: You need to study the mechanics and ensure that the ideals of your campaign will be upheld with these guidelines. Though once you are done, inevitably you will have a collaborative effort between story and system that will give you just what you need. Right?
Have you ever not
had a campaign where you houseruled something? Would you feel comfortable altering or even outright creating rules to a system that you aren't all that familiar with, especially when you don't know what prolonged and grand affects said change will have later on down the road? Putting a band-aid over a gunshot wound might stop the bleeding temporarily, but your arm will eventually fall off due to sepsis.
On the other hand, you can always use a system that you're familiar with and adjust the rules and mechanics to fit what you want to do in your campaign. Changing your basic D&D core ruleset into a modern apocalyptic scenario isn't all that difficult if you put your mind to it. There are rules for zero level characters and modern weaponry, plus you can use the Pathfinder variants for wounds instead of hit points. But then you'll probably want a fear and sanity metric in the game as well, just to make the characters freak out at the sight of their first zombie or when they have to shoot their best friend. But then what if you want to do futuristic combat and instead of zombies you want to fight ghosts, but you don't want the usual ghouls and geists that D&D provides, but more "realistic" ghosts that need pure iron and burning bones and ancient rituals? Once you start altering your system too much, is it really D&D anymore?
Ultimately, it depends on what your needs are and how much extra work you are willing to do in order to stay with something "comfortable." I use a basic 30% mentality, in that if I need to alter or completely change more than 30% of the ruleset then it really is not worth using that system, and it's my sign that I either need a different campaign idea or a different system. For me it's usually the former, however I'm sure at some point I will have an idea so outrageously out of the scope of D&D that I will develop a wandering eye and actually start reading all of those rulebooks that I've collected over the years. Who knows, maybe my players won't mind being elementary school children with over-protective imaginary friends.