Here are a couple of bullet points for discussion; most of them would have an effect on future diplomacy and late-game design

• An intro splash screen when starting a new game would go a long way in setting up the theme further:
  "You are the leader of your tribe in 5th century Europe /.../ The Roman Empire has split in Eastern and Western halfs in an attempt to govern it better /.../ It is an era of mass migration; your tribe has been swept in it as well, and is now looking for more fertile, richer lands to settle. But you are not the only ones knocking at the gates of Rome", etc :P
  
• The gift from Rome messages mention that the resources have been given for your service. Does that suggest that your tribe has already had first contact with Rome, and is receiving sporadic gifts/bribes to stay docile, keep trading peacefully, and remain a potential ally of Rome against other tribes? Since Rome is also willing to recruit your tribesmen as new legions when you train them, this appears to be the case. Three things follow from that:
  - The intro screen should mention some of this background info to provide the context
  - Since there has been some contact, there should be (at least) a rumour of the nearest Roman border fort indicated on the map
  - On first discovering Roman territory, the diplomatic screen/options should be different from the one used when first meeting another tribe (this may already be the case, I haven't yet encoutered either Roman faction in my playthrough)
  
• Rome is weakening through a series of semi-randomized events, as I understand. Do we get to hear some news about each in a notification?
  
• In case you play for an economic victory and at the same time do not extensively explore the map, there should be some additional ways to learn a bit more about the world. For instance, on forming an alliance, the ally should provide you with (some) of the map knowledge they have plus a few extra rumours
  
• On winning the game, a full map reveal would be cool, so that you can see what you missed and inspect the state of the in-game world at the end

What do you guys think?