If you're using XML, then you might find the following information useful for your mod.
First of all, buildings/wonders:
Code:
<TradeRouteLandDistanceModifier>50</TradeRouteLandDistanceModifier>
Increases the distance caravans can travel: one square for every multiple of 10.
Code:
<TradeRouteLandGoldBonus>200</TradeRouteLandGoldBonus>
Increases the amount of gold gained per trade route: 1 gold for every multiple of 100.
Code:
<NumTradeRouteBonus>1</NumTradeRouteBonus>
Adds one extra trade route.
Code:
<TradeRouteRecipientBonus>1</TradeRouteRecipientBonus>
Gains one extra gold for trade routes coming into the city.
[CODE<TradeRouteTargetBonus>1</TradeRouteTargetBonus>[/CODE]
Any civilization sending a trade route to the city gains one extra gold.
The Colossus and Petra automatically gain a cargo ship/caravan to go with their bonus trade route via this code:
Code:
<Building_FreeUnits>
<Row>
<BuildingType>BUILDING_COLOSSUS</BuildingType>
<UnitType>UNIT_CARGO_SHIP</UnitType>
<NumUnits>1</NumUnits>
</Row>
<Row>
<BuildingType>BUILDING_PETRA</BuildingType>
<UnitType>UNIT_CARAVAN</UnitType>
<NumUnits>1</NumUnits>
</Row>
</Building_FreeUnits>
The trade routes can also be modded into traits (as I'm sure you know

).
Venice gets double the number of trade routes like this:
Code:
<NumTradeRoutesModifier>100</NumTradeRoutesModifier>
In Anno Domini, I've used the figure 50 to give the Phoenicians a free trade route for every two trade routes they already have.
Code:
<TradeRouteResourceModifier>100</TradeRouteResourceModifier>
Resource diversity grants twice as much gold in trade routes for the civilization with this code.
Code:
<LandTradeRouteRangeBonus>5</LandTradeRouteRangeBonus>
Increases the range by 10% times the number.
Code:
<TradeReligionModifier>100</TradeReligionModifier>
Doubles the effect religion has in trade routes.
Finally, the Gateway to Africa's "Receives +3 gold and +1 [ICON_CULTURE] culture for each trade route with a different civ or city-state. The trade route owners receive +2 gold for each trade route sent to Morocco" can be modified - both numerically and by what it receives, e.g. it could get faith instead of gold. Here's the raw code - any "yield" is acceptable.
Code:
<Trait_YieldChangesPerTradePartner>
<Row>
<TraitType>TRAIT_GATEWAY_AFRICA</TraitType>
<YieldType>YIELD_GOLD</YieldType>
<Yield>3</Yield>
</Row>
<Row>
<TraitType>TRAIT_GATEWAY_AFRICA</TraitType>
<YieldType>YIELD_CULTURE</YieldType>
<Yield>1</Yield>
</Row>
</Trait_YieldChangesPerTradePartner>
<Trait_YieldChangesIncomingTradeRoute>
<Row>
<TraitType>TRAIT_GATEWAY_AFRICA</TraitType>
<YieldType>YIELD_GOLD</YieldType>
<Yield>2</Yield>
</Row>
</Trait_YieldChangesIncomingTradeRoute>