The 7 Seas would be an RPG (or possibly an MMORPG) instead of being the usual hack and slash action/strategy game, and would revolve around the world of pirates and pirate hunters.
Start out as a lowly deck swabber or port hand on either the side of the pirates or the side of those who hunt them.
The map would entail oceans, seas, countries and continents each with their own towns and ports. Each port and town would have their own style of economy that generates wealth and resources. Merchant ships would traveling around to the various ports picking up merchandise and gold/silver to trade with other ports.
The pirates would be able to raid towns and ports, merchant ships, and be able to attack the ships of pirate hunters who are trying to stop them. They could also either join up with other pirate ships for more power and divide the loot, or attack other pirate ships to keep the loot for themselves. The pirates could also set up bases on deserted islands or ports and towns that they have completely taken over. These bases could be used to re-supply, repair ships, store loot, build defenses, etc etc.
The pirate hunters would have the support of the ports and towns because they are defending them in a way by hunting down the pirates. They could start out with a home base and a ship and similar to the pirates and be able to attack and loot the pirate ships, plus either join up with other pirate hunters for added power, which would also mean splitting the loot, or attack other pirate hunters so that they have more loot and prestige for themselves without having to split/share it. As a pirate hunter with support of the towns and ports, you could set up your own merchant stores to generate supplies and money for your ship/s.
The ships on both sides would start out small and basic, with upgrades for the ships revolving around offense, defense, speed, etc. Ships can be sold and larger ships can be purchased to upgrade.
Pirates bases on islands or raided ports would also start out small and be upgraded with fortifications.
Pirate hunters would start with small bases within towns and be built up too.