Vassals

this is a bit old but how about in civ6 adding the vassal state concept?
A nation would become a vassal state to another by 3 ways:
a) when a much larger and powerful nation declares the intention to go to war against a much weaker opponent, one of the choices of diplomacy between the 2 tribes would be the option of the weaker becoming vassals of the stronger.
b ) when a nation is loosing bad to another in war and wants to end the conflict in order to survive they'd have a chance at becoming vassals of the victors
c) when at war with a nation, winning, but not wanting to spend more time and resources on the war you might offer them becoming vassals of yours.
Vassal states would/could be required to do the followingdetails would be negotiated between the tribes)
1. Supply a certain amount of military units to their overlords-per certain amount of turns
2. Provide a certain amount of gold and possibly resources, turn based as well
3. Sign a crippling trade(luxury) agreement with their new bosses.

The vassals would certainly try to get out of this state ASAP, especially when their superiors enter into a war with another nation, or by signing alliances against their (brutal) overlords.
Do you think this would work?

Actually your vassalage is close to the Civ4 one, and is very gamey.

In realitity vassalage comes from feudalism, and it was within countries most of the time. I wouldn't mind to see such a system in Civ6, but it would need people to live on their own as a third power.

It makes the most sense to allow you to vassalize city-states when conquering them.

It not only makes sense that vassals are the "lesser" nations that City States are supposed to represent but it would provide a much needed incentive to warmonger without a lot of the negative side effects (For both the warmonger and all the other civs).

It should also spice up diplomatic victory and the UN.

further thoughts on this:
1. vassal state would last 20 turns, or until war is renewed.
2. simplest way to brake the agreement by the vassal nation would be to fail to deliver the requested goods, money and soldiers by the end of the 20 turn limit.
3. at the signing of the agreement the vassal state would agree to one or more of the following:
a)deliver a specific number(type) of military units at the start and at the end(after 20 turns have passed) to the dominant side.
b) deliver desired luxury(ies) -at start and end of the agreement, of course
c) deliver the desired knowledge-technology
d) sign a completely unfair trade agreement-lasting 20 turns
e) surrender one of their cities

Also, IF YOU HAVE JUST SIGNED A VASSAL AGREEMENT AND THE OTHER TRIBE AGREES TO SEND YOU THEIR UNITS YOU COULD(AND PROBABLY WOULD SOMETIMES) DEMAND THEIR UNIQUE UNITS AS WELL. then you'd have other nation's uniques to rule for yourself, cool?

I'd really like Vassal states to make a return, or at the very least, have city states (assuming they make a return) be more aggressive in wars.

One thing I liked about the vassal system of Civilization IV is that it helped start some very big wars. With city states, sure, they declare war with you, but outside of a handful of cases, they don't really help you out to much.

also, when your nation is classified as a vassal state(the moment you sign the agreement in order to save your nation from conquest) your score would be frozen and points would be re-accumulated after you are declared a sovereign state once again

Previous attempts at vassal states were very artificial, and it has resulted in the current puppet system which I also dislike. I think to make vassals work you'd need:

1) a new internal diplomacy mechanic (which I'd love to see). It could take the form of squabbling lords in a kingdom (maybe 1 per city) or party politics in a democracy. In this case, it should act to give a sense of unhappiness over people under vassal rule (determined by tribute demanded, religion/ideology imposed, bribes from other civs to war etc.), and an ability to organise rebellion (see next point).

2) a civil war mechanic. Maybe on declaration (decided by unrest in 1)), a certain no. of cities and all the units created by them (or with units decided by some influence modifier determined by 1), maybe incorporating the ability to buy people out) form new colours and leader with the same unique stuff, and is at war until one side admits defeat, is conquered or there is an agreement to remain separate. It would be interesting to see how other civ's ability to side with one of these factions could play out.

3) much stronger civ equalisation mechanics for vassals that liberate (atm there are beaker discounts, but this is not nearly enough)

4) A big diplomacy hit for holding vassals of civs which are friends with other civs.

Having said that, I think this could add a lot to gameplay, and would also add to like a very closely related 'colony' mechanic (so I can have a colonial empire!).

I think the CIV IV Diplomacy mod handles this pretty good. I've encountered some weird situations (or bugs) but they weren't game breaking or anything.

They should reintroduce this with Civ Beyond Earth.