To define the time frame and the possible ships is probably one of the hardest things to decide on. What would be the biggest compromise between realism, maximum variety of ships and game balance is the question?
Some ideas to contribute to the brainstorming process
-for campaign and the normal game, why not take a rather later start date, e.g.1800-1810 with +/- 10 years as a benchmark for the ships that are available. This era should give a maximum amount of competitive vessels
Speaking of the campaign, this is independent from the political setting. I´m aware of the ongoing changing of borders/island ownership during that time, but to have an 1800 ship in a 1750 caribean setting will only bother realism fanatics. To have an 1800 ship in a 1530 geopolitical setting would be too much of course.
-for PvP only the possibility to offer ships from different eras, like a Mary Rose https://en.wikipedia.org/wiki/Mary_Rose, a Santa Maria https://en.wikipedia.org/wiki/Santa_Mar%C3%ADa_%28ship%29, a Brederode https://en.wikipedia.org/wiki/Dutch_ship_Brederode, the Russian Shtandard https://en.wikipedia.org/wiki/Shtandart_%28frigate%29 or the swedish Kronan https://en.wikipedia.org/wiki/Kronan_%28ship%29
– all mainstream ideas, but smart guys provided plenty of ideas in the sea trials sections, so there is no shortage of possible ships and ideas.
The ships from older eras probably won´t be competitive against more modern ones, but would offer some players or super pro´s (those guys that have achieved everything in game) a new flavor. These ships could be offered for special accomplishments or you could buy them for real cash – generating extra resources for the devs without endangering the balance and alienating the playerbase. Dedicated players would buy them for sure.