There is a mud based on historical Rome, and another I recall reading about some time ago which is/was set in ancient Greece. Using a "real-world" theme is certainly a viable option and could produce a good mud, but most people seem to prefer a fictional setting.
For example, taking your "pirate" mud, you've got the potential for an interesting game. But by adding magic, you would suddenly add a whole new element to the game. By adding monsters and supernatural races, you're vasting increasing the possible character and opponent options. In addition, making up your own setting allows you to "create" history - you don't have to worry about history buffs pointing out loopholes in your theme. Equally, you can add surprises for the players, because it's not possible for them to cheat and read up on what happens where and when.
So in short, while I think your suggestion could provide a fun and interesting mud, you would be losing out on a lot of elements which make muds popular. It's just something you should bare in mind.
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