I see three potential flashpoints so far, in order of what I think is most likely to happen.
Caliphate-Germany, over expansion and security through WISE 1506+7027 and Sigma Draconis arms. Depending on what sort of mindset Steve has for the Caliphate (or the Germans for that matter. . . ), I see imminent conflict between these two powers, with little potential for discourse considering they both really have nowhere else to go.
Japan-Centauri, over pretty much the same sort of issues I outlined above. This potential conflict is likely to drag in Mars (defense arrangements with the Fed, with the possibility of expansion deals with them), Argentina (Japan would like to make a deal with them, or the Federation will, for help or neutrality), and Manchuria, who would definitely jump on Japan on Earth if she starts to lose or the opportunity presents itself, though I am not sure if Manchuria can defeat the Japanese on Earth even if the Japanese lose in space.
Manchuria-Russia, over dominance, colonies, security or something else in the Russo-Manchurian arm they both inhabit. This one probably won't happen if Manchuria finds good territory to expand into, allowing both nations to expend their excess energy peacefully expanding. I will say a first strike on the other by either side would be very risky while most of their people, industry, and shipyards remain on Earth. Steve's NATO vs Soviet Campaign demonstrated what a few simple mistakes result in. All bets are off though if one side manages to find a position of, "acceptable losses".
What I am interested in seeing is what sort of response the other powers have to conflicts that do not directly involve them, but have the potential to badly damage them. For instance, the American and French response to war between Germany and the Caliphate, or any sort of large scale fighting on or near Earth by any power. Never forget the Trans-Newtonian campaign disaster.