Would be nice to request specific types of trade ships, especially when you start using mods that add all sorts of different kinds of ships. I like how it's done in the game Banished. You can request specific items and when the trader comes back you have to pay extra for those items. Rimworld already has cost modifiers in trading; traders already get a 1.4x increase on their sale prices. You could have another 1.4x increase for ships that were requested.
I'd really like to buy some slaves.
Recently I was adviced to try A Petition for Provisions (Request Items) (https://steamcommunity.com/sharedfiles/filedetails/?id=1904213444) for such thing.