I think that every Civ should have had Rome’s ability concerning trade posts, at least partially. Every new city settled should start with a trading post from that civ for refueling and monetary benefits. Rome can also get one in from conquered cities, but Rome will need a new ability.
Every trading post from a civ in a city should all add 1 gold. So, if a city has 10 trading post from 10 players the route gets 10 extra gold. Trade posts in each city along the route adds to it. As the game progresses, a good trade route may be worth hundreds of gold per turn.
Trading posts of allied civs can refuel your trader.
Traders should pay a toll when embarking and disembarking and that should cause trade routes to be more reluctant to choose embarking and disembarking when there is a slightly longer route if the trader continues in the same mode, land or sea. (If all other bonuses being equal between the routes) The toll should get steeper as the technology level of the trader progresses but the toll to disembark should rise faster and be initially more expensive. This will mean that once freight is on a boat, the trader will prefer the closest harbor to the destination with increasing intensity.
Traders should auto repeat their routes until canceling. Once cancelled it should end immediately and return, but not warp, to the closest city. You still get that route's proceeds until it returns, unless it gets plundered, and then it returns but provides no benefits while returning. Alternatively, there could also be an option to cancel when finished.
Wars and denouncements and other things which cancel a route should cause the trader to return, not warp, to the closest city. You still get that route's proceeds until it returns, unless it gets plundered, and then it returns but provides no benefits while returning. So that means that traders are not destroyed when plundered and traders never have to be rebuilt. (I don't know if that is a better idea. Some people might like the idea of sinking trade ships, but if they can be rebuilt immediately then it isn't that impactful and might actually be beneficial since you might immediately choose a better route).
Traders should be able to travel between airports. Realistically though they don’t ship very heavy cargo by air on long trade routes so that might be beyond a simplified system such as civ 6.
The multipliers for landmarks need to be adjusted upwardly significantly. trade routes nearer to world wonders should bend hard toward them, and perhaps from as far as 6 hexes but not to exceed some angle of deflection from the ultimate destination.
Mountain tunnels for traders should basically just be an alternative road rather than a warp.
Time savers and route shorteners like canals reduce the cost of trade route. How do you calculate that? This might take some coding and thought. Basically, there is a difference in distance between using the canal and not using it. I might redesign a trade route to charge a cost for every hex it passes through. Then passing through a canal simply saves money and time. Better roads lower the cost of passing through a hex. I don't this is quite right and things like coast and ocean could have different costs and costs could change in different eras independently of each other. This would take some thought.
edit: I would pose that ocean tiles are very expensive for a trader to traverse, much more than coastal tiles, when you first get cartography or ocean-going technology, but as boat technology improves, ocean tiles get as cheap as coastal tiles. So, at first, long distance routes over ocean might not be profitable if there is too much ocean between, but trading posts on foreign landmasses separated by ocean tiles could be worth 2 gold or have a multiplier for being on another land mass. So, you could run a long-distance ocean route earlier, but until you were able to pass through enough cities on the foreign land mass, it might not be worth much.
edit: Also, it would be cool if there were high magnetism for trade routes toward harbors and commercial hubs and traders would try to pass through these districts and would get bonuses for passing through.
These changes will transition the game more and more into a gold economy which will significantly speed up the end game. Since more and more building will simply be bought, production will go more and more into city projects which will reduce micromanagement of city build queues.
Building construction and maintenance costs will rise significantly as the ages progress to balance against the strengthening gold economy. Inflation multiplier. There should still be some decision making between what buildings to prioritize, but gold should be plentiful enough to allow the player to transition production to city projects. Maintenance costs that are not paid fully mean those buildings do not provide their bonuses in full. That happens before military automatically begins disbanding.