I think that the northern barbarian factions should all start out with special niches such as hiding and ambush with the Germanians, skirmishing with the Britons, and the Gauls with an initial balance of both in the early game.
As the game goes on, they should get armored and disciplined variants and become more similar to eachother, while still having their early specialties maintained and elaborated on.

I also think they should take longer to reach their best troops than any other faction. It would be a good way to portray the disorganization among the tribes, the difficulties in confederation, and the singular combatant nature of the barbarians; however, when they do finally reach their best and organize, they are frightening to see.

Britons:
I don't like how the Britons were in vanilla RTW, they were a gimmick faction without adequate ranged units despite being skirmish centered.
I don't think that chariots work well as the Briton's niche, a good special unit maybe, but they're unpredictable and clunky and don't do the faction justice.

I think that at the start they as a faction would be more timid in their engagements and prefer to pick battles favorable to them in a guerilla warfare style, and at the late game should get some of the best barbarian line infantry (who visually stand out, to intimidate their enemy and as a bonus reduce the chance of getting hit by friendlies) to take and hold ground for their capable ranged units to shoot from. (It would also be cool to see a druidic warlord unit similar to Germania's berserker.)

Germania:
I really like how Germania was done in vanilla RTW, their variety and gameplay is how all the other neighboring barbarian factions should have been like.
I'd love to see the German ambush gameplay made more integral to early game survival, with fearsome traps being able to be set up and casualties always trying to be minimized. At the late game they should have some of the most fearsome shock infantry and cavalry combined with dependable ambush units following around them (but with lighter ranged units) to drive home the fast moving and hard hitting style from vanilla.

Gauls:
They were usually the first barbarian faction you'd encounter when you played the game; they had a little bit of everything, and good religious buildings as well. I always viewed them as a broad barbarian faction with a jack of all trades, master of none mentality.

I think that by the end game, all three of these factions should get quality front line troops armored in chainmail in lower quantities, some overlap between these factions occasionally would be acceptable and they should play into their own faction's specialties.

I should clarify that I lost my copy of RTW a while ago and this mod is exactly the type I've always wanted to play it again, so some of my suggestions may already be in the mod, forgive me if so, and thanks for your work on this gem of a game.