Freeciv

These are little used because, I believe, missing a few things to really unleash them.

Let me praise the potential of policies.

A single slider represents a player decision, an input, which can potentially affect anything in the ruleset. In theory it could almost change the entire ruleset just from a single policy! This is be far the most abstract general power of anything I can imagine.

Things that are possible (I don't necessarily say I want to do them, Im trying to show the power here)
1. A game with no governments at all, the entire "government" is designed by policies, and the ability to do policies get unlocked by achieving certain things. Players literally design their own custom governments through policies!
2. A civilisation committing to special types of national character. Mongolian steppe riders can commit to more and more policies that unlock more and more other policies in time, making a whole different nation-race possible with totally new and different abilities.
3. The very nature of what a unit does and how actionenablers work, can be changed.
4. You could do things like, a Wonder that lets a Republic declare a Martial Law policy.
5. You could do things that increase costs of some things and decrease costs of others, give bonuses and penalties to elements all over the game.

In short this is potentially the single most powerful feature a ruleset designer could ever have.

But currently, it's so undocumented that I don't even know how to use it. And it appears to be "one way." I don't know, but it should:
1. Introduce the effect based on the slider value. I assume we can do simple true/false here just by having 0 and 1 for the values.
2. Have other effects be based on requirements for the policy slider.
3. Have the policy slider itself need requirements for you to access it. (e.g., only communist government can set a certain policy, )
4. Have the ability to set certain values have requirements.
5. Attach helptext/explanations to the policy and the different values.
6. Freeze a policy, by putting time-length restrictions in it. For example, once you choose policy x, then as long as you're a Republic, or as long as you don't have gunpowder, you can't change it back (through combining true and false for different requirements together.) example: you can choose to be the tribe of special archers or the tribe of horse riders or the tribe of legions who get bonuses for those but penalties for other things. Can't change the policy until gunpowder or some other requirement unlocks its frozen state.

If these policies have or had this two way logic to them, it blows my mind how many amazing things could be done with it. For now I'm asking what can we currently do? It's not so documented... and how hard would it be to make requirements "bi-directional" ... requirements to set a policy and other effects having requirements that there is a policy in place?

, you have a slider that represents a decision or policy that you as a leader make. The decision of the policy hooks into the ruleset which can change almost anything in the entire ruleset.