So, we've been all been playing for a couple weeks now, and we all know about the balance issues of the game, which Firaxis are reportedly fixing some of. One thing that I can can be easily fixed is Trade Routes. Here are my ideas:
International:
These are fine. The yields are comparable to what they were in CiV. One suggestion might be to let +% Intrigue gain or +% Operation success, either one way or both ways. Give a little more strategic basis for TR choice.
Station:
These are currently useless. +3 Food and +4 Culture really can't compete when International and Domestic TRs routinely get into double digits for all yields over sea. Stations need to have more tiers, ending up with higher yields, and they need to either a) Focus on a single, massive yield (+lots of Food, for example), or focus on Culture, which no other TRs can provide, or give other bonuses. Free tech was a bit too silly, but there's nothing wrong with getting a free unit or maybe some Affinity XP trickle. The important thing is to distinguish these from International and Domestic TRs, so that we actually have a reason to want to keep Stations around, and not just let them die so we can place cities.
Domestic:
These are the big offenders. They give far too much yield, they are too spammable, and yet there is zero transparency to them - why does on TR yield only food, and another only production? There doesn't seem to be rhyme or reason. To this end, I propose these changes:
1) Direction of TR does not matter, and does not change yields.
2) Domestic TRs grant 3 Food and Production for every 4 pop in the "outgoing" city. (i.e., a TR between a 6 pop and an 18 pop would have 12 F/P going to the 6, and 3 F/P going to the 18, regardless of base city of TR)
3) Sea routes get multiplied by 1.5.
This way, spamming small cities and sending TRs back to your capital won't actually do anything until they've grown to at least 4 pop, removing that "cheat." There can be balancing in terms of final numbers, but I think a move in this direction would be a good start.