How feasible (in terms of time needed to write the code) would it be to:
Gain a Traderoute between your largest 10 cities and each ally's largest 10 cities [only your largest city being paired-off with, then given the same benefits of a traderoute to, his largest city. . . NOT your largest city getting a traderoute to ALL TEN of his largest ]; and for those you have only a Treaty with, this is reduced to both players' top-4 cities.
...because it makes sense that a nation trades more (i.e. obtains far-away, rare-but-valuable resources, LEARNS due to cultural-exchange benefits, etc), if it finds & keeps allies.
If it's made simpler by the following, or this balances game-play, or is closer to what we know about real-world international trade: this can alternatively be done a myriad of ways, but two ideas:
1. pair-off & create a traderoute in each of the top-10 cities with the most TRADE COLLECTED PER TURN -- rather than using the top-10 LARGEST cities of the 2 civ's who've allied or signed a Treaty) ?
2. Instead of using top-10 cities of both players: Use an equasion that takes into account BOTH players' TOTAL trade-per-turn (the same way gold & science-bulbs are calculated... so this might be simpler than FINDING both civs' 10-most-powerful cities, then pairing them off & creating 10 trade-routes?) then distribute a bonus (similar to how the "Multiplayer Ruleset" lets MarcoPolo's Embassy create civization-wide 40% trade bonus...or at least a continent-wide 40% trade-bonus when using "Small Wonders") but apportion these bonuses to benefit the weaker civ in this treaty (or alliance) more than the "First World" (has discovered more techs, has more gold-income per capita...) civ so BOTH get a benefit compared to if they had no Treaty/Alliance, but the weaker nation becomes a net-exporter to the First World nation...to reflect the Real World "trade imbalances" in Developed/Post-Industrial nations --and the comparatively higher GDP-growth in most Developing Nations-- that we've seen in the 20th/21st century "Globalized Economy" ?