It would interesting to include a "Vassal " option in order to convince another major faction to join your faction, when your power level is high relative to theirs. The cost in Influence (and Dust) should be high, but it may be considered worth it if one can make an empire no longer a threat, and instead a stronger version of an Ally. Their empire would contribute 50% of the difference after paying their dust upkeep costs to you directly, as well as 75% of their generated Influence, until the end of the game after the vassalage agreement is concluded. The player also gets access to any techs the vassal faction has the player doesn't already have, as well as any new techs the faction happens to research later on (both cases including faction-specific techs that aren't irrelevant tot the faction, for example, food techs if the player is the Broken Lords). This vassalage agreement lasts until the end of the game and is unbreakable by any diplomatic or military means save if ALL of a vassal's cities are captured by enemy empires. Then, by definition, the vassalage agreement with that major faction ends. NB - In the main, aside from as described above, this operates as an Ally treaty, in that the AI retains control of their faction. However, they no longer build settler units, any current settler units they possess are disbanded, and they do attack or attempt to capture the cities of other factions. Lastly, there should be a bonus applied to a Diplomatic Victory win score for every major faction turned into a Vassal.