How about having a bar showing how the other empire is rating us. Any choice, settlement within ZOC, or other occasions will alter their view on us positive or negative.
Such a request of removing a space station in/near their ZOC will +/- some points on that bar. The more it comes into negative, the more harsh the diplomacy from them grows (and opposite). If it gets down to a certain level, waging war is possible. On the positive side, they could reach a level where joining us in an alliance gets possible.
The current system of smilies and plus/minus indicators is nice, but just a simple way to show in which parts they have tentions. Once embassys are installed, the bar could show up more detailed information on that bar.