The post pulls out a list of the “tools” Traveller Books 1-3 make available to the referee/GM. While the intent of the post is to try and shine a light on how Classic Traveller was intended to be played, I actually think it works great as guidelines for the minimal set of mechanics you’d need to build out for a system of your own devising if you want to give players the ability to run free-form games in the same spirit.
Boiling that list down into a format that works for our more generalized discussion here…
The theory here is that if a game has all of these (and not much more), and that the individual systems don’t make sweeping assumptions about the “canonical” universe that a campaign is meant to take place in, that these tools will be plenty for players to be able to construct a campaign of their own and find years of adventure in it.
Note that there’s a fairly even split there between rules for actually running the game, and rules for generating content. CT’s subsector generation and patron mechanics aren’t strictly necessary for moment-to-moment play, but provide a robust framework for a GM to structure a campaign around. Critically, they provide a few assumptions about the sort of game that the system is built for (it’s set in a small area of space, the player characters take jobs from people there, etc.), without burdening themselves with the notion of a canonical setting: as the Out of the Box series really drives home, the Third Imperium is entirely absent in Classic Traveller, and players are assumed to always be playing in a universe of their own design.
A quick note about “secret sauce”… In CT, these two sets of mechanics are starships and, far more briefly, psionics. I think this category is specifically for rules that are critical for running games of the sort that your system focuses on, and that wouldn’t necessarily be universally useful. As an example, a mecha RPG would almost certainly have mech construction/combat rules as their “secret sauce”. You can probably argue that, say, Mothership has stress/panic and, far more briefly, starship rules as their sauce.
In any case, thinking about mechanics this way has already changed how I’ve structured some of my ongoing projects. In one case, it’s made me realize that a game would probably benefit from some default mission generation rules. In another, it’s shown that the game’s rules are probably a lot farther along than I had originally thought. Hopefully, turning all this over in your head will help you, too! And if not, well, maybe it at least pointed you toward the Out of the Box series of articles, which is fantastic. :D