Another example for a rarely used crafting system would be the system used in Evil Islands, a somewhat unknown rpg from 2000.
Crafting was done only by NPCs for a fee, which would be in line with what the Devs for Sui Generis want. The NPC Crafter was able to build a few different very basic armors and weapons, i. e. a broadsword, a longsword or a two-handed sword. These recipes were always the same, but the difference would be the materials used, which the player can find in the game.
In the beginning you would build items with just some low-grade stone, wood, leather, strings or cloth. Later on you would still use the same recipes but you would build them out of better, more rare materials. So your first sword was made of stone and later you would use bone, obsidian, bronze, iron, steel and some more awesome or rare materials.
You could find those materials everywhere: as a drop from enemies, lying around on the ground, in crates or chests and also by dismantling other items.
However Evil Islands was very linear and consequently it was easy tying the available materials and recipes to the story progress. For Sui Generis there would have to be other obstacles like needing to find a certain crafting tool, location or NPC craftsman, which are only reachable with a certain difficulty. Simply placing the good crafting materials next to (or in) strong enemies isn't enough, because there is a certain type of player that will always be able to circumvent those easily without having to level up or train their skills. Either by being good enough to dodge everything the enemies have and slowly whittle them down or by just simply tricking or outrunning them. That is of course only necessary if most of world is accessible in an non linear fashion right from the beginning, as for example in the gothic games.
On a side note: The crafting system in Evil Islands had one serious flaw. Since magic spells could be incorporated as a 'material' in items, it was possible to stack heal spells on the items, that would be automatically cast by the player for no costs and therefor rendering the player nigh invincible. So better leave that out or restrict it heavily.
Anyway, such a crafting system would be relatively easy to implement, while having enough depth to satisfy crafters and at the same time being simple enough to not shift the main focus of the game away from the action.