Subsystems – self-contained rulesets that didn’t interact with the rest of the rules – ran amok in 1st Edition and, by 4th edition, have mostly been removed. There is still a place for the subsystem in D&D. I like introducing one-shot mechanics to spice up a single encounter. Oxymoronically, I like to use a consistent structure for all my subsystems.
I use a mechanic I call “Saving Throw with Fumble and Crit”. I’ve tried to invent a cool acronym for it, but all I’ve come up with is either “F On Toast” (“Fumble Or Natural Twenty On A Saving Throw”) or “Stoat Ass” (“Saving Throw, One and Twenty are Super Special”). Let me know if you can think of an even more unacceptable acronym.
The basis of every “F On Toast/Stoat Ass” subsystem is a chart like this:
Make a d20 roll, plus any situational modifiers.
1 or less (or natural 1): Critical failure
20+ (or natural 20): Critical success
The principles behind this chart are
a) that the 4e saving throw (essentially a coin flip that slightly favors the player) is a good generic mechanic, and
b) that “20 and 1 are Magic”.
I’ve used this subsystem template for my wilderness survival rules, mass combat rules, wandering monster rules, random treasure rules, and several other homegrown subsystems. It’s easy to explain to players, especially the second or third time the same structure is used. All you have to say is “Make a saving throw. Let me know if you crit or fumble.”