I don't give a damn about any of those things besides vassalage and unique great people, and even those aren't some sort of awesome necessity I'd put in the top 10 of desirable features
But again, gaming journalism is in the utterly desolate condition
Vassalage seems really cool, but I'm also not sure how would you implement that in civ context, where you have very few players in the map, and enormous problems with exponential snowballing making catching up impossible. Vassalage only makes sense if you can figure out how
1) Make it useful to the domineering party (instead of conquest or peace)
2) Make it useful to the dominated party (instead of fight at all cost always being better option)
3) Make it capable for the vassalized player to be realistically able to sometimes break free and catch up
4) Make it interesting for third party diplomacy (support independence, secret talks, espionage etc)
For me an easy* implementation based on civ VI rules* could be plausible, by the following lines:
*needing improvement on some areas, see below
Vasallage would basically a locked-in alliance, commanded by the "master" without option to disagree by the vassal, and limited diplomatic relevance of the second, which in turns supports the master. This means
- the master can fix the type of alliance he wants (cultural, economical, scientifical..., and the vassal would have to accept, breaking the alliance of the selected type if established.
- vassal would have to accept as well any type of diplomatic agreement that is bi-lateral (i.e, common open borders, or joint wars)
- 50% of the diplomatic favour gained by the vassal would be transferred to the master.
- suzerainity bonuses of city states would be shared, independently of who is the suzerain. (This reflects a city state that is vassal to the master identifies the vassal civ as part of the master subjects, and a city state that is vassal to the vassal civ is actually vassal to the master civ but ruled by proxy: remind 50% of the gained diplo. favour goes to the master, but in this case, the vassal civ retians 50% favour as its people are the ones who are managing things with the city state and still have some agency).
Here the domineering civs gets favor, suzerainity and alliance benefits (choosen by themselves), while the vassal civ still gets alliance benefits (altough maybe not the ones they would accept) and suzeranity, (and of course, living) just at the cost of diplo favour (= less international relevance). This might fit (1) and (2).
For (3) and (4) is were we need an improvement on current mechanics, and it would be extending grievance system to actually work: first, by making grievances extend to subjects beyond war, both agenda related and gameplay related. Here is one of the areas where I see Humankind has got a very interesting system surpassing civ.
Rejected deals should generate (minor) grievances. Settling to near or spreading your religion were your vassal has already his should generate grievances (it somewat does, but it should be more automatic). Stopping trade routes to a civ should generate (minor) grievances. Voting against proposals in the WC should generate grievances... Building wonders that the other Civ was also building should generate grievances (it generates grievance to you as a player, so the AI should have such a grievance too). Building any wonder in any case should generate grievances to Qin. Not spreading religion to him should generate grievances to Mbemva. Building cities on islands should generate Grievances to Gitarja... and so on.
Grievances should fill a bar (here is were humankind-type system enters), which allows you to Dennounce or Make War at no cost, and it is directly related to war weariness. For Vassals, there should be as well a treshold on this bar upon which they are enabled to declare independence (maybe you could as well declare independence without the treshold, in cases were your master is kind to you, but you will need to save the scarce diplo favour you retain for that). Of course, independence resorts to the former state: War, unless the master decides to let you go peacefully (gaining diplo favour by that decission). Masters can as well get grievances from the vassal, but they won't be able to do much with them, unless the vassal finally rebels and they can use them to justify penalties in the ensuing war.
This would cover (3), and for (4) it would be as easy as playing with this treshold: Master civ could spend envoys on you (instead of in city states), to increase the treshold at wich you can rebel (or the dipo. favour cost of the rebellion). Third parties may offer the party favour or use spies to lower the treshold / increase the existing grievances against the master. Spies / diplo visibility may be needed to realy be aware of the current grievance/treshold status.
Just a few ideas, probably some can be easiliy gamed or won't work as expected, but I think there is room for an interesting vassalage system.
Edit:
@Zegangani : please feel free to use any of these ideas in your mod concept, if they fit. I'm not used to Modding forum threads, and I was not able to figure how to follow the thread / put a meaningful post there.