I don't really feel the trade route idea makes sense for the Aztecs. After all, they have the boats able to cross the atlantic, and not anyone else, so the trade route is wherever they want to dock.
Here are some thoughts :
When the Interaction button does appear : after the first aztec explorers event
Who can interact : Anyone with a coastal province to the atlantic ( exact limit is up to balance)
What you can do to gain "grace" : Basically the same options as the chinese, with some variations :
-cash gift
-tributary (probably a different kind than the chinese one, less cash and more troops or something, Balance TBD)
-supply horses ( valid for everyone here, decrease horsey troops available)
-supply troops ( same but with all troops)
-offer a sacrifice ( i have no idea on how sacrifices were chosen, so i'll defer to someone more knowledgeable, but something like a prestigious vassal, a pious priest or a fair maiden would actually cause you trouble amongst your people, which is an interesting trade for a bunch of "grace")
-offer a concubine
-offer a commander
-offer a physician (if in plague status)
-send relief (if in plague status)
-offer artifact ( if appliable)
-offer to kneel before the emperor ( if appliable)
-Convert (large amount of "grace", plus a monthly gain)
What you gain from "grace" :
Here i'm not sure most of the current choices work.
To simulate the fact that the trade with them would introduce new resources and behaviors , i propose some boons of the following sort :
Trade in [New Resource] : small cost of "grace", monthly cost. Stop if you get to 0. small effect on character and provinces. For example :
Trade in tobacco : slight decrease to revolt risk /slight increase to land morale
If you have enough grace, you could even request to be taught about the crops you're importing, learning how to grow them. This would allow you to get a "growing resource" modifier in some of your provinces, giving a larger bonuses that could be expanded onto.
For example :
Learn how to grow tobacco : any 5 provinces get "Tobacco Growing", bonus to income, decrease to revolt risk, neighbouring provinces can build a "Smoking House", further decreasing the revolt risk
Other resources could be Cocoa, Corn, Cotton, ...
Some variation of the basic choices could still work :
Request a Commander
Request a Priest
Request an Imperial Mariage ( maybe)
Request an Artifact
A major difference between the chinese and Aztec Emperors would be that the Aztec one would be more active, and more prone to being irritated. You'll regularly get requests ( in event form) of one of the potential gifts, for a lower grace gain. If you refuse, your grace would drop.
( Note that we can use negative "grace" values in the code ). If it gets very low, it would mean that the Aztec Emperor is really irritated with you.
This would actually one of the possible causes of Sunset Invasion ( the other being the Empire feeling Expansionist). Basically, instead of being fully random, the invasion would be more likely to occur on people with low Aztec Grace and less likely on people with high Aztec Grace ( no risk of being invated if you're in the good grace of the Emperor and he's not feeling expansionist).
The statuses and policies then modify the likelihood of the various requests, along with the costs and gains of the boons and gifts.
Empire Status :
Stable
Civil War/Iroquois Wars/ Inca Wars : Requests of Troops more likely , less likely to invade
Plague : Grace drops heavily every month, less likely to invade, more likely to Request Physicians or Relief
Empire Polcy :
Default
Expansionist : More likely to invade
Religious : Common Religion raises grace faster, different religion lowers it faster. More Requests to Convert
These are just some basic ideas that need to be refined and expanded to make it more interesting but i feel it makes it different enough from the chinese system to be worth having.
(All i've described is a priori doable currently in engine without much trouble)