Start
All we know our normal castaways. Poor sailors whose ship got into some kind of trouble and they were left in the ocean to survive on piece of wood. But the only sailor-like ships we have in the game are Merchant ships, so where are merchants themself?
So why not Castaway Merchants?
About
Castaway Merchants will rarely replace normal castaways, and other than more fancier look and “Castaway Merchant” title they will also have a slightly different loot from normal castways.
For example instead random fish and 33% chance for treasure chart, Castaway Merchants will give you random item from pool of normal merchant, and with 33% chance they will give you additional item or more rarer item that is not usualy in Merchant’s pool. Perhaps even enchanted item.
Instead of 300 fame/bounty for saving / killing them, you will get 500. And you get slightly more galleons for escorting to safety / killing such a rich person.
Conclusion
The suggestion is very small, but it will make saving / killing castaways have slight chance of encountering a merchant that instead of some fish and some money, can give you an item and more money than normal castaway would.