Rivers would be very exciting! Hopefully they can probably flow and travel downhill rather than just being at sea level. Obviously a lot more complex, but seeing that would be awesome. Also having a variety, from smaller streams that are a few blocks wide and deep to larger rivers that are deep enough to sail big boats on would be great.
Excited to see procedural dungeons as well! I am very excited for this newly christened Project Glint of course but I do definitely want to experience and enjoy adventures within Vintage Story itself.