I can't see a 3rd expansion happening. Each expansion, as opposed to an update, is centred around some major new mechanic - era score, and then climate. I can't see any of the ideas proposed above being major in that way. Most of them are tweaks, and you can't build an expansion out of tweaks. Suitable for an update, though.
If you are looking for new mechanics, you just have to look at the mods available for both civ 5 and civ6.
Here are some examples:
- Interactive events, decisions and quests.
An interactive event system can be added (remember the interactions of Beyond Earth or the Outback scenario).
Events can be random or guided in a mission system similar to the victory system of Sid Meier's Civilization Beyond Earth.
It is a way to enhance the roleplay of your civilization as well as creating a story.
A lot of existing or news elements can be linked to this system (epithets, state religion, ethnicity, ideology,....)
Here are some pictures to get an idea (check the event and decisions for civ5, the JFD mods of Rule with Faith or all the wonderful work of Sukritact among others if you are interested in more examples).
- New Rankings, fairs, conventions and link these to, for example, the world congress.
For example, a scientific congress could be set up in the city with more science in the world, granting benefits to its owner, for the next call of the same, that city continues to come into play but with a penalty, etc.
Some pictures:
- New content related to new ages like Colonization Era or the Enlightenment Era some even fantastic or anachronistic like the future, prehistory or steampunk (Maybe the latter were a bit controversial, but they are fun)
To all this, there are a lot of options that have already been discussed in this thread as Corporations, revamp of trade, new disasters, plague and a mechanic of health, etc ...
In short, I do see room for many new mechanics.
Regards