I tend to play as Germanic tribes (Usually Burgundar) most of my campaigns and as already commented by others the campaign tends to be quiet passive (I know this has little to do with the mod and more with TW:Attila in general). I want to say though that I really like what you did with the Roman Empire which finally can hold out longer than 405AD. Also the Diocesis Britanniae changes the game a lot because you don't have to rush to get the territories the Celts left over.

I have some suggestions for a more unique campaign as Regna Barbarorum:

The goal: Constant war between the tribes, slow build up and a incentive to go for roman territory. In general more of a survival feeling to the game.
-> Ideally there should be be several phases: 1. Hold your own settlement and survive against outside tribes and tribes in your own province. 2. Go on quick raids into enemy territory and return to defend your settlement. 3. Raid other settlements. 4. Vassalize a neighbor and build up relations with them. 5. Actual conquest or migration into the Roman Empire.
Up to now: Good relations with all neighbors, sitting out for a large war chest and conquest only for "RPG" reasons. Also restricting myself from expansion for same "RPG" reasons.

Suggestions:
1. - Give the germanic tribes a huge garrison in cities too.
- Make standing armies very expensive. Especially higher tier troops should be much more expensive (up to 10times as expensive) so that the player has to rely on the unreliable tier1 troops with maybe 2-3 higher tier troops per army. 2 armies should only be maintained with low tier troops.
- Generally very high unrest. "Rebellions" of minor tribes in your province every 2-3 turns, starting with around 10 troops.
- Unrest can only be countered by a very costly and uneconomic "tribal holding" building chain. This way especially the player will be restricted to controlling one region only, because building two of these building doesn't make sense.
-> This is to make quick expansion difficult and to restrict the tribes to 1 settlement. Especially due to revolts.

2. - High propensity for AI germanic tribes to vassalize. The AI should almost always choose this option.
- Even higher and longer lasting unrest penalty for occupying a settlement.
- Terrible relations with vassals which very gradually get better. Mostly by helping them against their rebellions.
- Maybe high propensity for vassals to migrate.
-> This should prevent players and AI from forming large Empires to the north of the Rhine and Danube. Maybe the only way to actually take a settlement is to raze and resettle it. Also long standing vassals should be the only form of alliance you can make. (Building up such a settlement should be a crippling endeavor though too: even higher manpower cost, replenishment malus in that region and high unrest if resettled to early after razing). Also this should keep up the diversity of germanic tribes.

3. - Terrible relations between all germanic tribes (usually one historic exception for each faction). You should be at war with at least three or four different factions plus constant rebellion.
- Higher income from raiding. This should be the main income for germanic tribes.
-> War should be the primary source of income for the Regna Barbarorum and the Norse tribes. This should represent consolidation of power by that tribe while restricting it to fewer settlements.

Tech tree problem: If you want to you can get ambahti and comitatenses after about 20 turns. That is too early IMO. I think they should be part of the garrison earlier but recruitable much later.

I think the mod already goes in this direction in many points. Some things I would make more extreme or explicit. So similarly to EB buildings would represent different choices of how to structure your tribe. For example:

-From start -30 unrest per turn, but something like +20 happiness from tier3 chieftains holding which also gives some ambahti as garrison, but no new recruitable units. Cost for chieftains holding something around 10000denarii. The higher tier units should only be recruitable in very uneconomic smithies for instance.
-Passive income from economy should be much lower except if you focus on it which in return would mean less high disorder and lower tier units even in the garrison.

I know some thing are possibly unrealistic especially concerning balancing with the AI, but I wanted to summarize my thoughts on Regna Barbarorum campaigns (I still have some ideas). Maybe the EP team or other submodders find some of these ideas interesting.