p:. why would tax law be changing the game? ill bite
In most European cultures for most game period times, there should be no such thing as passive tax income from your vassals. I'm not talking about your demesne's income, but the automatic transfer of a percentage of income every month up the feudal chain. With Conclave, we got feudal obligations that always include tax. Pre-Conclave, CK2 was actually closer to reality with the default law being feudal vassals paying no passive tax. In any case, passive tax, like a modern "income tax" didn't really exist.
Taxes - of specific types - were levied at specific times for specific purposes, and the specifics varied from culture to culture. Some of these are actually semi-in-game. For example, when you or your heir gets married, you get a choice between a money bonus or a prestige bonus. That's a prime example of a "feudal aid" - one of the few ways the king would actually extract money from his vassals. In-game this is just treated as imaginary money that appears out of nowhere, but in practice it was a major source of feudal revenue. In practice, it was a constant tug-of-war between kings trying to extract more aids and nobles resisting, often violently. One of the major elements of the Magna Carta was to restrict the occasions when the king could extract feudal aids. The generally accepted ones, which CK2 should use or at least be based off of: when the king needs to pay his own ransom; when his heir comes of age; when his eldest daughter is married; and maybe when he goes on Crusade.
And of course there were other types of "taxes." Feudal "incidents" and "reliefs" were a large source, at least in England: when a lord inherited a fief, his overlord would generally expect a payment to "confirm" the inheritance. Of course, the noble could sometimes get away with not paying it, if the king were weak or the noble was in a position to somehow avoid paying. The amount would generally depend on the size of the fiefdoms inherited. In some situations, the king or lord would be able to hold on to the land (and gain the income from it) until the inheritor paid the relief.
There are plenty of other interesting examples. Many countries and rulers collected a danegeld, which was a tax on all landholders to either pay off Viking invaders, or to pay for an army to fight them off. Again, in CK2 terms, this was an event-based tax that began very infrequently (the first English one being collected in 991, and the second in 1002, and again in 1011), but that rulers would often try to collect on more and more occasions, with increasing resistance from landholders. By the 1050s Edward the Confessor tried to make it a permanent, regular tax, but was forced to discontinue by the nobles. (You can draw parallels to the later "scutage" tax as well, as it was based on land and levied for specific wars or campaigns, though there were attempts to convert it to a regular annual tax. By the 14th century the scutage was obsolete).
Judicial fines were also a huge source of royal revenue. In practice we already see this in CK2 via the sort-of indirect method of "well I can imprison this vassal for whatever reason, why don't I just do that and then ransom him." Also in some events where you demand a fine from a vassal.
These are of course all things that would also make peace time more interesting, since CK2 vassals are always dying and inheriting lands. AI traits would naturally affect how the AI would act in various taxable situations. Ultimately, if the game accurately represented many of the ways feudal lords collected revenue from vassals and tenants, it could eliminate the entirely abstracted "income taxes" that the game has now. This would also differ from culture to culture, including within Europe, and norms would differ as rulers established more regular tax practices, with increasing penalties for expanding them. Obviously the Iqta, Tribal, and Indian areas would need their own rules (and probably the ERE as well as it wasn't feudal).
(Now, that's not to say that by the 14th and 15th centuries, nobles and kigns didn't make a lot of money off of import duties and other commerce-based sources. But that should be a relatively late-breaking development that requires significantly larger and more profitable City buildings and merchant republics.)