About opening with Authority, I think Tradition is a solid alternative, and overall more synergistic with Japan's UA. The thing to consider is that the amount of GWAM points given by the UA is based on how much they currently cost; it gives 200 if it currently costs 400 (1 great person already born through points), and only 75 if it costs 150 (none born through points). So, by going Tradition and working those cultural specialists from Ancient Era, you get more GWAM points per GG or GA born than if you open with Authority.
Just in the example given, if you open Tradition and generate one Great Artist before getting your first Great General, those 200 points from one GG are worth almost 3 GG births from opening Authority, who can only work artist specialists in Late Medieval Era and is likely to not have generated a Great Artist beforehand.
It results in a different playstyle, since Tradition is not as geared for mass conquest as Authority is. Your units aren't as strong and capable of breaking through the enemy lines, so conquest ends being much slower. Opening Tradition gears you for a cultural victory, with wars being used for fueling up great work generation, instead of the usual warmongering rewards. Founding first is much more reliable, with that free +3 faith on the second policy.
When I go Tradition with Japan, I typically aim for Ceremonial Burial (culture and faith from expending GPs) instead of Hero Worship, since Tradition + Japan's UA can fuel it very fast and explosively. Conquest is optional here, and whatever I conquer often ends as a puppet, to avoid increasing culture costs that weaken those culture procs from Tradition and Ceremonial Burial.
Authority first, and then Imperialism later on, culminating in three different starts all ending in disaster, always falling way behind in tech and not being able to make any impact with an army, even with god of war, zealotry, hero worship and warrior monk picked as beliefs as well as wonders that should bolster things like terracotta etc. So this latest game I've tried a different tact; going Progress to get expansion up and running early and then Artistry to bolster great works and by god it's miles better!
I think Authority is currently underpowered, due to offering too little Science. This tree seems to consistently reach Medieval about 15 to 25 turns later than Tradition and Progress, which is a bad timing for the usual military spike that a human player can use to catch up with the AI, doubly so for Japan. If you don't snowball with Authority early you tend to fall behind, and you often need some early advantage (like an ancient UU) to get that snowball.
I've seen a few proposals over the last year that seem to want to drastically alter the kit, many of them fun alterations, especially the unique forge by @Legen, but I'm thinking now that might be going too far? Would love to hear peoples thoughts on this!
I think those proposals ended being too hasty, with not much time to think through the implications and what people actually wanted to see changed. The Torii proposals were under tuned to the point of being worse than just creating a regular great work, many people didn't want the Samurai to go away, and the voting scores show that people didn't want changes to the Dojo. People were also unenthusiastic about the Kabuki (at least those who voiced their opinions), and there was practically no discussion on the idea of Great Engineers proccing GWAM. We were trying to fix multiple things at once, and none of the proposals did so (if they even properly fixed) without trampling on things people wanted to preserve in Japan's kit.
In hindsight, I could have proposed the Tatara as a Kabuki replacement, simplified it, and left the Dojo and UA largely untouched (the common factor among the two most voted proposals). If it were now, I'd have done the Tatara proposal along these lines:
- UA: Yields on Defense buildings removed.
- Kabuki replaced by Tatara, an Unique Forge with the following design:
UB - Tatara (replaces Forge):
Unlocked at Bronze Working
Cost: 90 
(from 110 )
Maintenance: -1 
Iron: +1
+2 
Culture and +2 
Science (from +1
Science)
Enables the construction of Yorishiro: Sword, Yorishiro: Jewel and Yorishiro: Mirror.
<< maybe insert additional effect >>
Sword
1 of 3 Mutually Exclusive Unique Japanese Buildings.
Cost: 20
Allow
Food and
Production Internal Trade Routes to be sent from this city.
3
Writing slots (
+15 theming bonus,
+10 Production if themed, requires 3 Japanese works, any era).
TR to or from this City generate
50 Production to the origin City when completed (same as Kabuki).
Jewel
1 of 3 Mutually Exclusive Unique Japanese Buildings.
Cost: 20
Allow
Food and
Production Internal Trade Routes to be sent from this city.
3
Art/Artifact slots (
+15 theming bonus,
+10 Culture if themed, requires 3 Japanese works, any era).
TR to or from this City generate
50 Culture to the origin City when completed.
Mirror
1 of 3 Mutually Exclusive Unique Japanese Buildings.
Cost: 20
Allow
Food and
Production Internal Trade Routes to be sent from this city.
3
Music slots (
+25 theming bonus, +20 Science if themed, requires 3 Japanese works, any era).
TR to or from this City generate
50 Science to the origin City when completed.
Basically, still a stronger Forge that gives +1 Iron, and with a twist on the Kabuki's internal trade route proc, which adds variety and new considerations to the early game. Also, strong theming bonuses akin to world wonders, and for all three GW types; this solves the issue of not having great work slots for early UA procs and gives this UB a clear synergy with the UA. There is room for an extra effect from removing the yields on Defense buildings from the UA; maybe something like each yorishiro giving a +1
Faith and +1 of its respective yield (
e.g. +1
Production for Sword) to great works in the city. I wouldn't try adding the Engineer idea back now, given how little attention it got during that session, nor the Kabuki's GWAM birth procs, with how little enthusiasm it received. Maybe people's opinions have changed by now, though.
I could also think of a better Torii, but this one is more complicated. The scaling is very different, since you need to account for both great work and improvement scaling, as well as decide whether the Torii is supposed to scale in relation to the theming bonus of regular cultural buildings (
e.g. Museum) or to World Wonders (otherwise, it's better to create regular GWs if you built, say, the Great Library or the Notre Dame). I had some ideas, but I'm not enthusiastic of how high the yields can be (over 60 in the late game) just to be on par with a GW + improvement. It also requires new DLL code (if just for the GWAM bulb scaling), which I don't want to propose after all this time the 4UC integration has gone through.
In terms of possible changes, I wonder if a source of iron attached to walls could be reasonable/feasible? +1 iron from walls would make Samurai very consistent, no idea if there is any historical justification for it though.
Nice idea although this is where I think the Iron would be more accurately applied to a forge since historically it was their Samurai swords which were so expertly wrought.
The Tatara's idea was based on Japan having documented technology to extract iron (and eventually steel) from ironsand, and doing it in a scale that is economically significant, since the 6th century. By contrast, the West first invented a way to do it in the 18th century and stated to be the first time it was ever done, unaware that Japanese smiths have been doing that for more than a millennium. This timeline gives a sound reasoning for an early unique building that gives extra Iron. And thinking now, given that it had important economic, cultural and religious implications within Japan, it makes sense for the Tatara to also have bonuses to internal trade routes.
CS, 80 
RCS
Movement
GAP on Construction
