There were two quick thoughts from the Sins 1 diplomacy model that came to me earlier and that I wanted to post now while the Sins 2 diplomacy is still in its early days. I kept the post title generic in case others want to come in with their ideas but I suppose the inevitable specialist threads will appear once it is time.
1. Sins 1 had a very binary check for whether there were border tensions or not and it was a pretty steep malus iirc. For Sins 2 it would be nice if that were more granular. For instance, a minor malus for each controlled planets 2 gravity wells apart (i.e. a neutral/third party well in between) and a moderate malus for each planet directly adjacent to another players. Depending on what eventually happens with minor factions, the malus could be lower for indirectly controlled planets, proving a soft incentive to keep buffer zones between empires if you don't want to fully antagonise a major faction (yet).
2. It would be nice if the fleet strength parameter were more explicit in the direction of its impact. When I got back from breaks playing Sins 1 I often had to remind myself if that parameter was very negative because I was too weak (and ergo easy prey) or because I was too strong (and the other player was afraid of me). Either could conceivably work but let's make the description clear of what it is.
Also, in line with my other thread around better visual breakdown of fleets, it would be nice to have bars to indicate the strength or the influence factors. Or order them by importance. In Sins 1 there were quite a few factors so that it wasn't always easy to tell from a glance what was driving the overall value.