I don't understand this aversion to fictional characters, even in areas we have no record of rulers we generally have a pretty good grasp on what was going on there at the time. Whats so wrong with adding someone made up to to fill in a blank?
I understand that for many people it might be hard to understand. But let me try to explain my point from several points of view. And let me tell you that it's far beyond aversion against fictional characters.
I did several mods and researched and filled the map with characters myself... which probably brought me to some sort of aversion towards made-up placeholders. I mean it's fun if you make up one or two dynasties, or several rulers for a dynasty which you know was there, but not when you fill 30th county next to each other this way and you need to do that for 150+ years for each. Then it's annoying beyond imagination.
We can use lists of historical characters and extend them by semi-historical or legendary ones (still plausible characters, many of which can get really fun*), but from my experience even if you stretch the timelines by pro-longing their lives to almost ridiculously unrealistic lengths, you will hardly get to year 900-950 in large areas of the map. There are many where you can get even little farther back, but OTOH in other areas you only know about a tribe living there and one or two rulers for the entire timeline. For 867 it's often very hard, for any date before that it is almost impossible and you end up with what I described above. I prefer if this game would be more about historical or at least semi-legendary characters rather than completely random ones.
* and I mean it's fun reading about them, creating them for the game/mod as well as then playing them
I don't consider this the primary issue though.
The main issue for me is that the devs of this game have dedicated 4 years on developing a game which is - as they say - designed around feudal lords and their world. The graphics -the look of things, the immersion, everything is set to certain time period. For propper simulation of the entire world it also requires well-done simulation of their contemporary non-feudal societies, but studying medieval history of various parts of the world I realized that all those people with their various systems and variations of feudal, imperial, or tribal societies all used very similar technologies etc.
In the earlier start dates you won't find any of this. Before Charlemagnes reforms, you won't have a single feudal realm, the tribal or semi-feudal societies of late 9th and 10th centuries were already closer to their contemporary feudal societies than they were to tribal societies of 700's. And since I am a maps lover, I can't ignore the fact that this game uses static map based on static system of de-jure countries. And the world in 1066-1400 despite its own dynamisms and minor changes was somehow stable, but for anything before 900 you need very different map. For pushing it before Charlemagne's administrative reforms, you'd need to completely re-paint the entire map of Europe - it can either fit the era of 700-900, or 1000-1400, you simply can't have both
I absolutely love the period of the "Dark ages" - the transformation of Late-antiquity societies into pre-feudal and later into propper feudal systems, the mobility and dynamics of that era, the reforms and energy of the Byzantine world, the energy and absorbtion of Ancient world's cultures and knowledge into Islamic medieval civilization in the 8th and 9th centuries...
but the way I see this game to be heading, it simply feels like different era. I'd rather have the earlier era being depicted by total-conversion mod for I:R. Perhaps even total conversion mod for CK3 could work, but not the same game with same graphics, map, lifestyle etc.
A separate game would be the best solution IMHO.