Most sailing games treat ships just like cars on water. I’d like to play a game with more realistic sailing mechanics. You control a ship with multiple sails which you can furl and unfurl, some of them turn, etc. and you have a rudder and an anchor. You control the ship by giving orders to your crew, like a real captain. You don’t have a GPS-style map. Instead, you must use “paper” maps, a compass, a sextant/the stars, and keeping track of time and your speed to determine your position and find your way to your destination.
Wind and weather conditions change over time, meaning you have to adjust. If it’s cloudy, you won’t be able to use the stars to navigate. If the wind is blowing opposite of where you’re headed, you’ll have to learn to sail against it. If a storm rolls in, you’ll have to choose between waiting it out safely in port, probably losing money, or risking everything to test your skills sailing through treacherous waters.
Framing all this, I imagine a career mode where you start off as the captain of a small merchant ship. Buy goods as cheaply as you can and transport them to where they’ll sell for more. Or take on contracts to deliver cargo or passengers for a bigger payout at the risk of harsh pay cuts if you don’t manage to deliver on time. Dangerous routes will pay out more, but you might lose everything if your luck turns bad or your skills/crew let you down.
Use your earnings to pay your crew and keep your ship supplied and repaired, with profits going to upgrades. Add whaling boats for a new source of income, for example. Or outfit your ship with cannons to defend against pirates, or to do some pirating of your own if things get desperate. Eventually, you can save up to buy better ships, which will be faster and hold more cargo but also be more complex and challenging to command.
Managing your crew could be another element to deepen the immersion. At a base level, you need to keep them paid. Making sure they’re supplied with rations of food and rum will keep them happy. Over time, they will gain experience and become better at their jobs but also demand higher pay and better conditions. You can choose to fire them and hire new, cheaper sailors but they might fail you in a crisis when you need them most. Allowing them to be too unhappy for too long could mean losing your ship to mutiny.