Higher level trade goods could be unlocked with certain traveling traders. So there would have to be a way for them to keep coming back. If a Trader is killed by enemy races that upgrade is lost. I have no idea what Hearthling traders from the three kingdoms would sell. Maybe Undead traders would sell magical jewelry and weapons?
Rabbit Trader - Greens Merchant sells grain, foodstuff (turnips, beets, potatoes, beans, peanuts).
Level 1 - Wortelbrau recipe carrot beer (for Brewmaster)
Level 2 - Tannic Acid & Wood Soap (for high level Carpenter furniture), Wood Ash Lye (for tanning leather)
Level 3 - Boomerang blueprint (Archer sidegrade) Rabbits use boomerangs instead of bows
Level 4 - Papyrus & Ink (for Alchemist, Magma Smith, Geomancer, Engineer)
Dwarf Trader - Traveling Alchemist sells ore, ingots, minerals and chemicals.
Level 1 - Rum recipe (for Brewmaster)
Level 2 - Gunpowder recipe (for Alchemist), Bombard grenade blueprint (for Alchemist)
Level 3 - Blunderbuss blueprint (Archer upgrade) Dwarves use blunderbuss instead of bows
Level 4 - Phlogiston bombard recipe (for Alchemist) basically fireball spell
Goblin Trader - Shifty Tinker sells discounted (stolen) weapons, armor, etc.
Level 1 - Goblin made pots and pans (for the Cook)
Level 2 - Tanglevine trap (for Big Game Hunter) non lethal trap, Fishing Rod (for Hunter to become Fisherman)
Level 3 - Javelin blueprint (Archer sidegrade) Goblins use javelins instead of bows
Level 4 - Stink Bomb blueprint (for Alchemist) causes enemies to flee in fear