Setting this as a 1.17 goal, as I do not want to change synergy calculations mid-version. The categories system will be added to the mod immediately.

What Should Be Added or Changed?

Categories for materials, which will be used in synergy calculations. Default categories will be things like "metal" or "gem". Any string entered into the JSON files for a material's categories will become a new category, so other mods and pack developers can add as many as they want. Materials with matching categories will mix well together, while different categories will reduce synergy.

Materials can have multiple categories. How exactly this will be used in the calculations, I am not sure. Perhaps it would be best to just say if any category matches, consider the whole thing a match. The weight of rarity in synergy calculations should be greatly reduced, or maybe even removed entirely. The weight of tiers should possibly be reduced as well.

(edit) One possible solution is to reduce synergy if no single category matches every material in a part (eg two gems and a non-gem would give the penalty), and give small bonuses for additional matches (eg three gems, but two are also organic). This would make avoiding the penalty relatively easy, while rewarding players who want to go the extra mile and fine-tune their combination.

Why Should This Be Done?

Allows greater control over synergy for pack developers and provides a more intuitive way of matching up materials for synergy.