This game has received an update.
https://steamcommunity.com/app/859580/discussions/0/4358996214438789472/And also a second one.
https://steamcommunity.com/app/859580/discussions/0/7845908402041401038/So, I complained about this game as not being up to it's contemporaries in Stellaris or Crusader Kings 2.5. However, right after that there was a final patch that improved quite a lot of problems. After that development ceased because it already had a bad reputation. However there was a mod called Invictus that has kept the game alive by essentially picking up development in mostly the same direction. It's a solid game with Invictus mod. One bad part imo is that development ceased at a time when Paradox games were designed to have some sort of highly limited resource like "political influence" in Stellaris used to be where the whole game is waiting for that one resource to crawl up at .15 points per month so you can do much of anything. Imperator still maintains that period of Paradox mechanics and to me that's the biggest f'n grind in this game because everything uses it and sometimes in amounts that take a game decade to accumulate while also having to use it for 8 other high priority reasons like setting governor policies to avoid loyalty loss for 6 months to a year of income of this resource, on your many individual governors, and each and every time they have a successor. I really wish that one in particular was set to the province rather than the governor character, because building crucial cities costs oodles of the same damn resource. Plus it can be used to fuel income on Stability for 5 years, and that's what limits tribal migrations to convert pops instantly and spread the blob to unclaimed territories for free land and pops easy. Otherwise, except for that single resource being a big limiter for a diverse amount of reasons, it's enjoyable.
Also, here's part of the expanded map from Invictus mod. It adds some culture groups and many small cultures that even have a paragraph or two of description in their tooltip:
In the base game I did tribals with the baltic culture (where Aestia is in the image). The tribal culture I started as (Black Hoods? I think, it shared the culture group of the baltic cultures, Aestinian or something) has as it's cultural bonus a +2% or +5% attrition penalty in it's lands This and a similar ability granted by a state deity would be the best option due to tribal hindrances against a techhed up Rome-blob with a war goal directly on me, this tribal can get I think around 10% baseline monthly attrition in ideal conditions with those two but it might be lower. and can retreat slightly faster, so the 500 size cavalry units that are faster and can't be merged with other levies as tribals can retreat before being wiped when I made that mistake. The 500 light cavalry levies are great for pillaging Rome though.
I do enjoy the migration for tribals even though it's very micromanagement intensive and I wish I could automate it so that when a county has enough pop it migrates automatically, resettles with 1 or two (depending if the county is currently producing same culture and religion population since if it is not currently producing same culture/religion pop then 2 same culture/religion pops are required to be settled for the NEXT pop produced after the current one to be same culture/religion as the owner of the county). It basically nullifies the culture and religious difference penalties for tribals once you know how to use it as cheaply as possible to avoid revolts and demote pesky upper class pops down to tribal class pops of the same culture and religion. I'd probably need to use a time limit extender (which Invictus has as a submod) to civilize to a Republic or Empire since the centralization-thingy in this game goes from -100% to 100% and just plain crawls increasing from -100% as some of the centralizing laws are blocked by a minimum level of centralized in the +20% to +40% range or somewhere around there. At -100% centralized the Stability cost for a tribal migration is only 5% though, which is outstanding since that's maybe 10 months of waiting in between and I can keep Stability floored due to having used migration conversion making those penalties surmountable. It's doable unless I'm trying to expand by fighting a war which requires a minimum of 20% stability to start. The biggest headache I had with migrations was when barbarians settle a land I pulled the population from for migrations and outnumbered the same culture/religion for long enough to tank the province loyalty, at which point you can't use migration there to convert all the pops dropped off when barbarians pillage from the impassable barbarian stronghold "counties" and you are likely going to a civil war or revolt that usually is wider spread than the province that causes it. To prevent that I'd try to keep more pops there in those provinces than normal and leave the governor on conversion tasks, and keep a province to re-raise a migration from after moving up to 11 same culture/religion pops and 10 of the differing pops there, that is the limit for a single migration and will leave a single pop behind (usually but not always the same culture/religion) ; less than 21+ pop counties can be raised but it leaves the county unclaimed with 0 pops and destroys any buildings, I'm not sure if other things are lost on unclaimed status but I didn't notice any offhand so I still use no-building provinces for less migrations from counties with less than 21 pops. That way only corrupt or inept governors cause the civil war threat, and provinces stay loyal enough to raise non-conforming pops into a migration to the wilderness or later to the cities finally being constructed once all the unclaimed land has been migrated onto.
In the game the screenshot is from, I allied to the Scythians early on to prevent them from invading me as I migrated into blob-form. They fought a few wars with Izygia and helped me seize some land in that direction in a defensive war IIRC, but also was usually allied to them against Alania. I fought few aggressive wars, most of the territory is from settling unclaimed land and converting the locals with migrations to send to next unclaimed province once Stability creeps up to 5% for a migration. I sent migrations out to unclaimed land in other places (it's how I managed to have elephants) as far as Gaul, Africa, Yemen, a few passes in the Himalayas, and even a county or two depopulated by the ceaseless civil wars in what used to be the Maurya Empire in India. These usually revolt away due to distance and my appalling level of Stability, but if I win the war they become some type of Vassal relationship if I remember correctly, and that's something Tribals otherwise struggle with as they can't do much of that without some tech innovations if I understand it correctly. Anyways though, those little tributaries have a military guarantee, but Rome doesn't care and likes to invade them anyways. That allows me to send small, fast groups of raiders into Italy or to try to grind down their manpower pool with attrition if they try to take some land, but I wasn't that lucky in that game and they usually beelined the tributary with a deathstack of legions to get the warscore without having to go on 8% to 10% baseline attrition before factoring in mountain pass fortress sieges, and so I hired some mercenaries to run into their stacks rather than risk my own manpower pool in a head on fight against legions since they probably have better logistics than I do as a tribal and I'd be the one taking heavier attrition. Finally, as a tribal you can outright seize counties in a unique way by raising a migration wave prior to declaring war. During the war you can settle an opponents' occupied counties and seize them for an instant flip to your control, no war resolution requried. I did that a bit but since the wars were defensive I didn't have a bunch of resources pooled to do a big wave of pops seizing a whole province at a time from Rome and had to utilize pops already on migration at the time of the war declaration.