What if, after Augustus conquered Germania, he initiated plans to build a wall across the most easily defensible border on the east, possibly from the Vistula river to the Black Sea?
Of course the wall would probably take longer than he would live, but if its completed and maintained, what effect would this have on the Roman Empire?
Would it successfully stop the influx of tribes? Would Rome last longer? I'd assume they'd be able to hold onto what they have a strengthen said hold, or would they just stagnate?