The AI instinctively knows when it can join a coalition, something that the player has to be constantly looking around for. My suggestion is that we get a little notification flag (green priority) with the coalition symbol to tell us if it is possible to join a coalition against any nation.
To go along with this, having a red notification flag pop up that you can mouse over to tell you who is in a coalition against you would also be very welcome.
I realise that this is probably the wrong time to make this suggestion, as you Swedes are off on your holidays right now, but alas.
I did a (poor quality)photoshopPaint3D of what I mean:
This is actually a bloody brilliant suggestion @Lambert2191 !!! Opening my suggestion file ASAP.