Liberating minor civs

Nice idea. Alternatively, (perhaps with a future espionage system) undermine the influence of a major faction over a minor faction, pushing them towards independence. 

Also once a minor civ has been liberated it gets a 10/20 turn immunity to seiging from other major factions to avoid being bogged down chasing and defending minor civs rather than fighting major civs

Another idea that might mesh with this: declaring war on a minor faction, invading them and especially assimilating them through force would have a significantly detrimental effect on relations with major factions that are friendly/allied with said minor faction, more so for an empire that has suzerainty over the minor faction. So, essentially a pledge of protection, same as in Civ5.