First, a preface: I'm generally not super-interested in the "what is historical" circlejerk, I think ultimately all Paradox games are about representing a fantasy of a certain period and fun should stand above realism. That said, I think for that very reason it would be better to go with the "tribes were more organized than given credit for"-argument as laid out in this.
There's two core things that I would change about tribes:
a) They need some sort of raiding mechanic similar to Crusader Kings. This would make them more viable, since they can leech money - and possibly tech? - from nations better at making those resources; while empires would need to be more concerned with defending their borders, which would then also mean that they can't concentrate all their military forces on a singular conflict.
b) The whole problem of a tribal "kingdom". The way it works right now, tribes are this weird inbetweeny kind of system between monarchy and republic, where rulers reign for life, bu dynastic succession simply doesn't happen, which I feel there is no real basis for.
Here is how I feel a tribal monarchy vs. a "civilized" monarchy should be represented:
- A civilized monarchy has established a certain sense of statehood. So there is always, say, a somewhat defined Kingdom of Macedon, and succession crises are about who is the owner of that kingdom. The default state would be a dynastic succession, but this gives rise to pretenders, based on familial relations. It can also lead to a particular weak king (say a child needing a regent) with some actor from within the centralized state structure (like an influential military commander) assuming power for himself. (perfectly illustrated through the wars of the Diadochi)
- A tribal monarchy is still much more based on interpersonal relationships and direct loyalty to the ruler, rather to the concept of kingship itself. So a succession crisis is very much about who the various tribes accept as their leader.
So how would this work in gameplay terms?
I think a tribal kingdom should essentially be immune from Civil War. Rather than that, disloyal tribal chiefs just break away with a part of the territory after some time if you can't make them loyal again (so no treshold here either) and you get a claim on them. Dynastic succession would be possible and you might gain some perks for it, but every possible successor (so the heir of the current leader as well as other tribal chiefs) has a rating for how respected they are, the less support they have, the more chiefs will break away on succession, and so you might instead decided to pass power to another family that can keep the kingdom together.
In turn, it would also be possible for other independent tribes to submit willingly to yours if you're particularly renowned leader.
When you annex another tribe in whatever way, you get the choice to either make their current leader a new tribal chief (improving the output of the territory and giving them a bit of loyalty), with one of your existing Clans getting demoted; or you give it to an existing clan who might be more loyal, but giving big maluses to the territory, who will not immediately accept their new rulers. Tribal leaders offering their submission themselves would obviously always demand to become a new clan chief.
On the culture level this would also mean that tribes get improved conversion in their own core territory where they are the dominant culture; while clan chiefs would eventually start to represent other cultures that are then immune against getting integrated.
It would require to either change how levies are calculated for tribes - they would get them from unintegrated cultures as long as they have a loyal tribal chief representing that culture - or just have multicultural tribes represented as vassal-swarms in the future.
This would solve two problems at once: it would buff tribes while they are united and enable major tribal coalitions against others (being close to an expansive power like Rome could raise the chances for tribes to willingly submit to other tribes); while simultaneously keeping them distinct from monarchies.