I would love for there to be very large islands or a deep “coastline” at the edge of a side or corner of the map mimicking a continental coastline.
There could be several rivers leading inland, with an outpost (or outposts if large enough) on the coastline at the mouth of a river. The rivers lead into a deep web of other rivers and offshoots, like the Amazon Basin.
This part of the map (whether a big coastline or multiple very large islands) would introduce two new concepts: rowboats and inland fort “ruins.”
Rowboats must be taken upstream because you can’t take a ship. Rowboats can haul loot quickly. The only way to get to the fort ruins is by taking a rowboat. Rowboats would then become part of the world at large, allowing you to park off the shore and get loot to and fro more speedily.
Fort “ruins” would simply be what we now know as skull forts but with new design and mechanics unique to the landlocked layout. These ruins could be guarded by skeletons, cannibals, or a new supernatural faction. B***y traps and action-oriented puzzles requiring teamwork could be part of the design to help differentiate it from the current skull forts.
Thoughts? Ideas to add? I’m thinking long-term and pulling from my personal wishlist. I have no clue if Rare has ever commented on something like this, but I’d love to see a balanced and properly implemented version of this at some point.
EDIT1: I'd also like to add the idea of the rivers, deltas, and tributaries being a treacherous habitat full of dense jungle and danger. I'd love to possibly see piranha in the rivers, alligators on the shores, and panthers and/or boars stalking the jungle so that leaving your rowboat becomes a fight for survival to make it out of the jungle alive.
What happens to your ship on your expedition is an interesting and important question, and whatever the solution I don't think this question is a reason not to consider an idea like this because it could add so much to the world and gameplay.