Technically, the different servers should also represent different, far geographical locations you can't reach by foot.
So, wouldn't make sense if there were some differences in what you were able to gain in these servers?
What i propose is to introduce some crops and forageables that can grow just in some servers, or maybe have very reduced gains in the others. Thus favoring exporting of the local goods and the importing of the rare, costly (since you've got to consider the cost of the traveling) foreign items.