I think the core issue is rather the opposite: people over-emphasize the way that so-called "tribes" (I really hate that term) differ from so-called "civilized" (that one too) folks and just start concocting a bunch of arbitrary limitations to "tribes" that don't really make any sense. Oftentimes the reason one civilization did something and another didn't boils down to context; one civilization found itself in a situation where that thing was a reasonable and viable option for dealing with a specific issue, the other didn't. Standing armies is a good example of this; Rome didn't adopt standing armies because they were better and cooler and more powerful; it adopted standing armies as a solution the the very specific problem of a dwindling smallholder population creating a huge manpower shortage for the levy, which was itself a culmination of several compounding factors stretching back decades and even centuries that were related to things specific to Rome's history. State-provided equipment and training was a solution to the specific logistical problems of a standing army that's drafted primarily from the poor who can't afford to train and equip themselves. Standardized equipment and training was a solution to the problem of ballooning cost since now the state and generals have to foot the bill rather than the soldiers themselves.
This is just completely wrong. I suggest you read up on the
background of the sacking of Rome by the Senones a little over 80 years before the start of the game. Below are some excerpts from the Wikipedia page, emphasis added:
The Celts here actually appear very level-headed and diplomatic. They are justifiably incensed by a violation of diplomatic norms (diplomats violating neutrality) and seek only that the violators be turned over to them for punishment. It's only after the Romans respond by elevating those diplomats to high offices that the Celts, rightfully livid, then proceed to take up arms against Rome. The war was entirely based on a diplomatic insult. Does that sound like a group that doesn't have any rules of war to you? And this plays out constantly throughout history amongst so-called "tribes". Also warscore should generally be based mainly on defeating armies rather than occupation, not just for tribes. A Tribal Warfare CB was a silly idea in EUIV and it would be a silly idea here. Everyone should need claims of some kind to go to war; in fact, tribes should be
more reliant on war justifications because their force projection capabilities should be highly reliant on convincing their people to fight for them because they have few-to-no institutions that give them the ability to induce or coerce populations to fight for them.
In some few cases, I'd like to see the free level one fort on capital removed, but without the ability for armies to garrison settlements a few erroneous free forts on the margins are a perfectly reasonable abstraction. Also, there would be very few cases where it would apply; keep in mind that this time period coincides with a significant rise in fortified and walled settlements in Northern Europe. So pass on this.
This is the one area where I kind-of agree with you. I don't like anything past "tribal retinues should have ... different rules to levies", and I wouldn't really say they should be
completely different. Keep in mind, there's virtually no information about "iron age Northern European conscription systems" to speak of, so this is conjecture on my part, but I imagine the conscription system would look more-or-less like how the Greeks in the Iliad formed their army - on the basis of a complex chain of personal relationships and ad-hoc oaths. The end result would probably look fairly similar to a levy-based system: the army would be formed primarily of smallholders who could afford to arm and train themselves and headed by the wealthiest and their well-equipped, well-trained personally-employed warriors.
I would just get rid of diplo slots entirely. I think it's a terrible mechanic. Also, everyone should be able to sign non-aggression pacts with everyone. Hard pass on limiting diplomacy amongst different culture groups; in fact, I think diplomacy should be made even less restrictive for everyone and it should be balanced by actual balance-of-power mechanics and actually reasonable and realistic limitations to growth and conquest. Definitely passing on this condottieri idea.
This I agree with, but - as evident by the OP - there are so many misconceptions about the topic, and it's already an extremely niche academic topic with little information to go on, and Paradox has a rather annoying habit of just going by popular misconception with no critical analysis whatsoever that whenever they do get around to reworking tribes they probably won't do them any justice whatsoever and we'll be back at square one until the next rework, eternally and ad infinitum.