As far as I can tell in vanilla and most mods, the only "flavor" text is first contact. Each LH has a couple of unique phrases, and there are no "generic" first contact phrases. Between vanilla, warlords and BTS there are about 30 LH (whatever the exact number is), so there are 60-90 phrases. Each phrase is given a text entry like AI_DIPLO_FIRST_CONTACT_LEADER_AUGUSTUS_CAESAR_1 in file Assets/XML/GameInfo/CIV4DiplomacyInfos.xml. Each phrase can be restricted to only certain civilizations or only certain leaderheads; in practice, for first contact, each phrase is restricted to a single leader. When it needs a first contact phrase, the game searches for all the "available" phrases and then randomly picks one.
The text itself is defined (in all four languages) in three files in Assets/XML/Text: vanilla CIV4DiplomacyText.xml, warlords CIV4GameText_Warlords_Diplomacy.xml, and bts CIV4GameText_DiplomacyText_BTS.xml.
What I did was remove all the leader restrictions, so that each Fury Road first contact picks any one of all the first contact phrases. Some of the phrases are very recognizable ("I'm always looking for CLOSER relations...") but I have not tried to weed any out.
So, if you wanted to give 2-3 first contact phrases (or more) for each leader, I could easily plug them into the files. I don't think we need to worry about other types of phrases. Have you seen other mods that customize other types of phrases?