in a peace treaty it should be possible to negotiate the exchange of cities, treat borders, impose forms of government, and create new state and political entities, limit armaments, create vassal states, cede colonies, and impose economies and economic treaties
All of this is more or less already in, or has been in past iterations, in one form or another. For example the AI is more willing to trade one or several of its cities in peace treaties when they have already lost some or they feel they lost the war.
Treat borders however would be too complicated for what would just be stealing a luxury or a strategic resource, because the player is not patriot and just wants to win.
Impose form of government has already been done IIRC, but it was kind of useless, would have been very useful in Civ5 with Ideologies however for example, if it wasn't possible (don't remember). It should have an impact on the gameplay, and as of now, government types for other civs is is the range of "I don't care". (except if you want to befriend/ally, but knowing the defeated AIs denounce you short after the peace treaty, such a choice wouldn't be coherent, unless there is change here, which I would be all favourable to it = make your old foe an ally !! However that could be like being a warmonger and have tons of allies, so maybe not in the devs agenda)
Create new states and political entities, basically new civs in the terms of the game, seems difficult to achieve ; and for what purpose ? There should be a whole range of new mechanics to justify it from a gameplay stand point.
Limit armament even in the reality it wasn't all white or black, instead the game would put a limit on one's army for X turns, and it would be impossible for that player to create more than X amount of units. The contrary of European countries "allowing" (tolerating by default) Hitler to rearm Germany in the 30's whereas he wasn't supposed to do it. Additionnally I find it interesting for a beat civ to have a chance to "revive" somehow, or recover from its hounds, which is practically never the case in the facts.
Create vassal states we had it in Civ4, which was good, but maybe underused maybe for lack of interest ? And in Civ5 we had puppets. Nothing in Civ6, it's very disappointing, yes.
Cede colonies there should be actual colonies labelled as so to mean anything in the actual state, unless we are in Civ3 with corruption and the AI settled a crappy city near your borders, that could be seen as "colony" by the AI in peace treaties, but even so, I doubt any player wouldn't have go for this city ASAP.
Impose economies same with government, and economic treaties means also the AI will not denounce you 3 turns after the peace treaty, or changes in the denouncing mechanic should intervene.
Economic treaties we have them in a way, it is to say gold/gold per turn as what could be seen as "reparation". But I agree, this is linked to city trade as in most of the wars when signing treaties it was about to "give back" the conquered cities to the loser, in exchange of gold per turn and maybe flat gold too. Obviously this is less a concern for Deity AI with all its bonus gold, but still would be an incentive for "reviving" defeated civs. (in the state of the game : without their capitals... could we change this ? Like vassals would count as "conquered", but could still win the game ?? But what if they enter in direct competition with you ? Unvassalize and raze or proper conquer/puppet ?)