I think multiple trade routes to and from specific cities should be limited, it makes sense and is good for balance. A simple way to do it would be making a hard limit on trade routes. To ensure there's no conflict between internal and foreign trade routes, the limit should be for both. So if the city limit is 2, they can actually have 4 trade routes (2 internal and 2 external). The limit should also count towards both ingoing and outgoing trade routes, meaning you can't send every trade route to your capital with the highest bonusses. Also trading with the best foreign cities will be a matter of who came first. I suggest the limit per city should be: +1 for city center +1 for palace +1 for harbor and commercial districts Policies, government, leader abilities etc. could also influence the number.