I take it the Germanic tribes of this period wouldn't have had a monetary system and most tribes would have become wealthy through raiding/conquering other tribes. How about a system where a tribe can only build something in their territories depending on how many battles they have fought. For example, a small battle against less than 200 rebel troops will let you build something that costs up to 800, whereas defeating a full Roman stack will let you build something costing much more. Or you could use the battlepoints (or however you wanted to describe them) to build more troops, bigger armies would require constant battles to keep the army together. This would encourage players striking deep into enemy territory, I think the Sweboz have raiding traits which makes this less of a problem than with other factions.

This would probably need the whole monetary system to be controlled by a game-master and a lot of console use to control the more abstract system in-game. I could draw up a proper set of rules if anybody thinks it could work.