I've noticed that there isn't any real incentive to trade with nations in your alliance, or your allies. The alliance trade tab seems almost pointless at the moment.
To encourage trade with alliance members and allies alike, my suggestion is to introduce a trade loss penalty.
Give everyone an inherent base of -15% (or -10% for a softer effect) when purchasing resources on the global market.
For example, Nation A purchases 1,000 iron from the global market, but only 850 makes it to the nation. The rest is lost due to piracy, theft, and/or accidents. The buyer bears the responsibility of the loss.
To offset this penalty, a nation could do several things, including:
New National Project: Harbour (or Port Authority?)
The Harbour increases the efficiency of your docks, whilst reducing the number of accidents. Trade Loss Penalty reduced by 5%. (I have no idea how much it should cost lol)
The security from trading with nations within your alliance ensures cargo is protected and arrives safely. Trade Loss Penalty reduced by 10% (combined with the one above reduces it to 0%).
A trade treaty with another alliance/nation helps ensure your cargo is protected and arrives safely. Trade Loss Penalty reduced by 5% (or 10%). Limit the amount of trade treaties allowed per nation/alliance to prevent abuse. This also wouldn't stack with the above perk, but it would with the national project.
Obviously, the trade penalty reduction wouldn't be able to be reduced past 0%. This would incentivise trade within alliances/blocs, creating more tight-knit communities, but it may increase the price of resources across the board.
Anyway, that's the end of my suggestion, be kind, thank you.