Sometimes for a session you find you want to try, or need to try, to handle something that your system of choice just doesn't prepare you for. Perhaps you're running a game and want to have a crafting competition that the PCs are engaged in. Or perhaps your PCs are beginning a business or criminal enterprise. Perhaps the PCs are getting involved in politics and you need a way to manage influence across factions.
This is the kind of thing where in a videogame you'd go into a mini-game or minisystem where you're almost playing something else in order to manage it. And when it comes to your RPG you can do that too. However, unless you have something to steal from another game that just works with your system of choice, that often means you have to design something. And with that comes the risks of getting in your own way of the game.
I find myself doing that right now for one of my D&D games. So let's talk about some pitfalls to watch out for.
Remember What Your Game Is About
Remember what your game is about. Yes, it can be fun to build a system for handling deep, intricate politics in a world. But if your game is about adventurers going on travels you need to keep that in mind. That is what your players signed up for. That is what you signed up for. Be careful before changing that.
A Simple Tool Is Probably Best
Speaking of that, and assuming that this isn't going to be the main focus of your game for the rest of time - or foreseeable future - remember you don't need a complex tool. You just need a tool that can get the job done. That game about adventurers? It doesn't need a 40 page politics rulebook unless you are now a game about political factions and ruling a kingdom. And since you don't need that, don't build that.
You Can Just Fake It
Never underestimate how much can be done on the spot with improvisation and simple tools. Things like the Forged in the Dark Clocks + skill checks can cover a lot of ground for handling longer term projects. A couple of random tables for events can give the illusion of a deep and complex setup for how the world is set to work. Your players don't need to know it is all bubblegum and duct tape holding it together, as long as you have the means to track things as you go forward.
Don't Go Against The Grain
Finally, try not to go against the grain of your system. Some systems are better or worse at certain things. D&D 5e for example, is a horrible system RAW for a survival game, because there are just so many ways to avoid the mechanics and drama of survival with a certain group. That doesn't mean you can't have a survival section, but any toolkit you build for it (or even trying to use the one in the DMG) is going to struggle to bring drama or tension because a lot of things just come with handwave mechanics to avoid the struggle.
Instead, try to work with your game. Try to latch onto the abstract tools already in place to leverage things. Temporary wounds can be a wonderful way in a lot of systems to denote granular fatigue if you don't have a mechanic for it - or if the mechanic uses bigger steps than you want to use. Just remember that fatigue heals faster than most wounds and you're good to go.