Diplomacy oriented game rules?

[Suicide Junkie]

You could try making a mod where all of the happiness types have lower bonuses / stronger penalties for all kinds of fighting.
A reduction of the martial law bonus too.

That way, you'd have to have limited wars, or your economy would collapse as the people start to riot and leave you with nothing but orbital factories and stored resources to run your military on.

Perhaps a bonus to happiness if you lose ships, (the "rock and a hard place" effect) so that the poor sap being pounded has an advantage, and players can't get wiped out so quickly.