[Q]

And make speech have an effect on the mood of AI empires!

In SE IV you can send whatever insulting messages you want to an AI empire without effect. Of course I realize that you cannot really ask for the AI to analyze the message itself, but if you only had some major categories of general messages like

- pleading
- insulting
- threatening
- friendly

and some more and specify for each AI the reaction to it, you might add some more flavour to the game.
I think this was already planned for SE IV with the "message tone" but as far as I know this has no effect at all.