do you think this mod is close to a final and stable version or is still a subject to future updates?
That is a fantastic question. In the short term (next 2-3 weeks) my plan is to keep some bandwidth open to deal with any bugs people find in the update. Previous versions had an embarrassing quantity of bugs. I hope this update will have less issues (since I was able to do more rigorous testing on it and the scope of the changes were much smaller) but to quote Dijkstra "Testing shows the presence, not the absence of bugs".
After people have had some time to adjust their strategies to the new policies I will probably release another update that will refine any rough spots that were created by the "On a Sword and a Prayer" update. Numbers may get tweaked up or down. If any ability ends up being problematic it could be replaced. A few policies that have become bloated may get hit by the editor's knife.
Longer term, I may experiment with a complete replacement for the Rationalism tree. But at this time frame (at least 2-3 months) the plan is pretty flexible.
In conclusion, this moment represents a point of temporary stability for Reform and Rule. However, it does not mean the mod is now set in stone.
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Piety
Ordained Priesthood does not make Great Prophets cheaper. There are some technical limitations that make this tricky to do with Social Policies.
As you keenly observed, there is a tension between a Missionary approach and a Great Prophet approach. Do you find one approach to be superior to the other (given the current benefits the policies provide)?
There are parts of Ordained Priesthood and Mandate of Heaven that provide value even if a player is following the opposite (Missionary vs Great Prophet) approach. Do you just wish these parts were stronger? (Example: +25% Science from the Grand Temple instead of +4).
There was suppose to be a subtle link where "Great Prophet = Tall" and "Missionary = Wide". I could play that aspect up more.
As for Reformation, do you see it as the policy you would always pick last (if you were completing the Piety tree)? Or is your concern more that it lacks synergy or the flavor of it is unintuitive? My thinking behind Reformation was that if Pious empires are not building Monuments than they need other sources of Culture to help them fill out their policies.
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Exploration
With respect to the exploration finisher/tree could you elaborated on your view of it? I have a vision for the Exploration tree but I don't think it is being communicated well. Exploration is the most likely tree to get adjustments to it in the next version.
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Liberty
Do you find the finisher so weak you don't bother completing the tree? If so, what policies do you skip?
In the past (back in the G&K days) Liberty's finisher was +1 production for all resources. It got toned down to its current version because it was pretty easy to get 2-3 luxuries in every city. An alternate version of Liberty's finisher that I've considered using is "+1 prod from bonus resources, +1 gold from luxury resources". It isn't strictly better (because in general production is more valuable than gold) but the gold gained will be greater than the production lost.