I agree that Civ IV diplomacy was probably the best in the series, but it also had issues of being too stale, due to diplomatic modifiers tending to compound and to the significance of religions. Being friendly allowed more trading and less wars, which made you more friendly, and so on. Religion basically determined whether you had the positive cascade or the negative one, and you ended up with fairly static religious blocks pitted against each other. Even after people switched to free religion, the accumulated peace and trading benefits of having once shared a religion kept such friendships stable. I actually like both the significance of religion for diplomacy and the fact that friendships and rivalries tend to increase over time unless something major changes, but I think that combination created for a stale diplomatic game when religions almost never changed. I think the game should have some mechanics that cause religious changes that shake up the existing diplomatic order, like a reformation mechanic that splits an existing religion. For example, you could start a new religion by reforming an existing one. This would give later founded religions a chance to thrive and become significant by allowing them to piggyback on the existing beliefs and followers of an earlier-founded religion. By increasing how often religious changes occurred, it would also shake up diplomacy.