I may have suggested this before. But we do war quite a bit. We don't trade like we could and it's kinda outdated imo.
So my suggestion Is. Alliances tarrif other alliances. Example being TKR puts a 5% tarrif on Pantheon. Any Pantheon nation buying goods from TKR has to (automatically of course) pay 5% more on the sale price of goods.
This could be a neat thing to add to war terms as an ingame political tactic.
We have nation Embargoes. We should have tarriffs.
Also some way we can (encourage) trade amongst allies or in alliance members
Say a 3% Ally discount or 5% Alliance discount. Where you pay a reduced price for the goods to better encourage trade