As many have said, trading with elves looses it's interest after a while. They have very little that you can use after a while, and the "please don't cut down trees" can get on ones nerves.
I know the elves are getting some improvements, and was wondering is some sort of rare herbs would be a good idea. Elves are supposedl much closer to nature, so it would make sense they would also trade with goods of nature. Since logging is not their way, and food and cloth is produced in abundancy in dwarf fortress, they could sell herbs that dwarwes can't harvest, or grow. They could have very expensive herbs to sell, which could help speed up recovery, repair damage to organs, or even help regrow a new eye or arm. This would make them very important in trade, and forcing player to choose between chopping down all the trees or limiting the chopping but getting to buy the herbs in exchange.
As for diplomats and nobles, I would like foreign diplomats (and possible heroes, nobilities) to come visit their counter parts after dwarf economy starts. They would bring along servants, and friend, who could spend their money in your dwarf shops. This way your fortress could create money in trade without caravans. Sure, it would be in terms of foreing money, but since that money is made of precious metals, it would still have value. Richer travelers could even stay in your fortress for a while, spending their money before leaving. Paying for food, bed and shopping (since there is no entertainment to pay for) with gold that is inserted in your economy.
Travelling heroes could do the same. Every RPG player knows that after killing the dragon and taking it's treasure, some R&R with good food and possibility to buy new fancy stuff at the "city" is always welcome.