@Obskiuras If the idea is to create a vanilla compatible civ (500BC-1BC), then I would strongly suggest to model this civ on the early Germanic peoples of the Cimbrian War, which was a major conflict that involved the Romans, Gaul and the Iberian Peninsula. Then I would focus specifically on the Cimbri, Teutons, Ambrones and even a splash of Helvetii (Celtic allied forces like the Tigurini). The buildings should then be modeled on the archaeology of the Northern German and Danish Iron Age. See Genava's earlier references, and the references in this thread on Cimbri, Ambrones & Teutons. If it's to be vanilla-compatible, Germanics should be all thatch imo, perhaps one or two plank-roofs, while Gauls can have more wooden shingled roofs to get that opulent Celtic feeling, but Germanics shouldn't feel more opulent than Gauls. Thatch for the Germans and shingles for the Gauls. Maybe the Germans could have a single structure with shingled roof: the "Helvetii Embassy", to train some La Tene Period heavy swordsmen.
If it doesn't need to be vanilla compatible, then you could easily focus on the later Germanic peoples of the Migration Period. If you want Germanic stone architecture like the one's you referenced, I'd suggest checking out Millenium AD which already features the Carolingians (they also have the Norse). Perhaps they're interested in a Merovingian faction as well? Vandal Kingdom? Gothic Kingdoms?
By the way, I really like you concept arts... Please continue.