I would argue at the very least, considering we're running on the honour system anyway, communications be limited to in-game unless players have publicly announced an alliance/achieved full co-operation (+50 relations) in-game or are at war in-game. This would reassure players that they are not disadvantaged in forming alliances and people aren't actively plotting against them without them being able to see it coming (i.e. their enemy just allied with another player, or a big alliance is brewing). But it would also allow co-ordination between allies and some more complex peace treaties. If I think my opponent is winning but can recognize that a larger threat is looming for him, it would be nice to be able to explain that in some detail, as a few lines in a public chat are unlikely to change my enemies mind about attacking me, creating fairly generic warfare and fairly standard outcomes.
In one game I am currently playing as Carthage I went the usual route of attacking Sicily. In hindsight, I think an interesting play could be to agree not to attack in exchange for a long-term alliance that would see Sicily become a buffer against Rome for Carthage (while freeing up their troops to deal with any problems in Africa quickly and secure a more stable foothold in Spain), in exchange for their ability to survive. This dynamic would be really interesting in a game and create cool variations in outcomes, but would be largely impossible to negotiate/co-ordinate without some at least limited private chatting.
Indeed in that scenario Sicily could probably rush Tarentum with fleet support from Carthage. This would create a very interesting map and situation for Rome. Now that I think of it, I really wish I could go back in time and change my choices
