I've been working on changing the Knowledge tree. I don't like having a tree that just supports Science. Science is useful for everyone all the time no matter what strategy you are employing. You never neglect building libraries and universities for example. I would prefer if all the policy trees were situational. Sometimes very strong for particular strategies, and sometimes next to useless for other strategy. So with that in mind I've been overhauling the tree.
Here's what I've been testing so far:
Opener: +3 science on national wonders.
Sovereignty: +2 free Gatling Guns. +50% production for building Arsenals and Military Bases. +2 food on Arsenals and Military Bases.
Scientific Revolution: +1 food on farms.
Humanism: +50% Great Person rate. +1 free Great Engineer.
Free Thought: Double yields on Great Person tile improvements (academies, custom houses, etc).
Secularism: +2 Science per Specialist (unchanged from original policy)
Finisher: +1 free Great Scientist, +1 free Great Engineer, +1 free Great Merchant.
The tree is designed to be good for small empires with large cities. It's unlocked when a civilization reaches the Renaissance Era. It's meant to be an alternative to the Exploration tree, which is clearly for large, spread-out empires. The early trees of Tradition and Liberty are designed in the same way, and Knowledge vs Exploration is a continuation of that design.
It seems like the game is a bit biased now towards large, wide empires over small, tall empires. National Wonders are meant as a game mechanic to help level the playing field. Too many of these national wonders are available too early in the game in my opinion. It makes it too easy for a wide empire to quickly build those national wonders, then expand rapidly (as a wide empire does). This way, they get the benefits of both tall and wide, which is poor game design. For that reason I've also modded some of the national wonders and when they are unlocked in the tech tree. All of the national wonders are unlocked in the Medieval and Renaissance eras. Heroic Epic is unlocked with Physics (an offensive military tech). National College is unlocked with Education instead of Philosophy. National Epic is unlocked at Theology and also requires a Theatre in every city instead of just a monument. Those 3 are the early national wonders. The other ones remain where they are.
In short, national wonders should be built only by tall empires. Otherwise, they can easily be overwhelmed by their larger neighbors. The opener policy of the newly designed Knowledge tree is meant to be a dud policy for wide empires (who won't have many, if any national wonders). The policies in the tree concentrate on things that are very useful for a tall empire. Specialists, lots of stuff with great people, great people tile improvements, defense buildings to create mega fortress cities, a couple defensive units. The food on farms is very powerful for a tall empire. The tree is overall very strong, but it's a late tree so that's okay.
I played a test game with Babylon going with a tall empire strategy. Those policies were very useful. It was awesome having all these academies (since Babylon tends to have lots of them) with each one yielding 16 science. The extra food on farms helped the cities grow really large. Grassland tiles were netting 5 food with a farm. 2 food for being a grassland, 1 more from having a farm, 1 more once Civil Service or Fertilizer is researched, and 1 more food from the policy. The extra food for Arsenals and Military Bases is designed to make those buildings worth having. I never seemed to ever build them. You would only need them for a pretty large city, which is what tall empires have so I put some bonuses on them in the Knowledge tree.
Also, I think the Wealth tree needs some work too, but I'm not sure exactly what yet. It should probably be a counterweight to the Aesthetics tree, which is also unlocked in the Medieval era. It's probably okay as a gold, production, and military tree. The Aesthetics tree is for peaceful, culturally empires, so maybe Wealth should be for more aggressive expansionist empires. I do tend to use the Wealth tree most when I'm playing wide and aggressive. The one policy that stands out needing to be changed is that Mercenary army policy. I never, ever use Landskenects (spelling?) Maybe that policy would be better being a more general military policy.