Currently, Diplomatic Victory is like voting a player as the winner which makes it seem kind of hollow and undermines the concept of 'allies'. It should require all remaining factions to be allied and, once this happens, then they all win (rather than just one). Then it will be something players might actually work towards in multiplayer as they do not chance losing.
To keep the option of winning without killing everyone or having others win, I prepose there be a Domination Victory. This will require all remaining players be made into vassel nations. Achieving this can be done through multiple methods, making it a hybrid victory.
Conditions to make a vassel:
Military) Defeat their sovereign in battle (even in their own land) and have a significantly stronger military (equivalant to the highest military bonus for AI relations).
Diplomacy) Having enough diplomatic capital compared to the faction you want to make into a vassel. Then you can force them to be a vassel.
Additional ideas / better ideas are welcome. They need to have a condition that shows you are supirior to the other faction and an action that causes them to become a vassel.
To cease being a vassel, you need to overcome all the vasselizing conditions (military stregth, diplomatic capital, ect).
Being a vassel would be like having an alliance, a peace treaty, and one way versions of the economic treaties (tech & gold) that only help the non-vassel.