Another thing is the fact that even if they did go through with a campaign to conquer Magna Germania, the relative unattractiveness of the land would make it less desirable for roman colonisation than the areas along the "Rhenus". It seems likely that they would go for looser control, which in EB can be represented by limiting those areas to "alliances available".

Another thing is that if we are to consider what the romans would consider the natural limits of their domains, it seems a bit strange that Britannia is "subjugation available" while Hibernia is "alliances available". It is just a short sea journey from Britannia, and probably would not be more difficult to annex and romanise than Britannia was, assuming the romans had desired to do so.