I'm sure this has been done to death but since it has been bugging me so much, just posting it here will, if nothing else, ease my mind.
For this though I will be changing more then just adding a ship, for I think the Galleon is perfect where it sits at the top. No, we don't need anything higher. Anything higher should only be regulated to an event-like ship.
First and foremost, rename the Sloop and Brig, you know, to what they actually are. The Sloop will become the Cog, and the Brig will become the Sloop. Sure it might be confusing at first but I'm sure people will get used to it, and, you know, not confuse people about what those ships ACTUALLY look like.
Secondly, allow the Galleon to have up to 5 people. I personally believe each ship (the current Sloop and Brig have this) should have a full broadside and Helmsman manned at the same time. Adding another player to the Galleon just meshes cause the new Brig will have up to 4. So the new player count would be Cog: 1-2 Sloop: 2-3 Brig: 3-4 and Galleon: 4-5 recommended.
Now onto the new ship. The Brig, recommended between 3 and 4 people, a 3 cannon broadside, 2 masts, and 1 lower deck. The look will be quite simple. It will have an Aftercastle like like the Galleon where the Wheel will sit, but has no Forecastle, looking like the front of a Sloop instead. Underneath the Aftercastle will be a captains room. Smaller than the Galleon's and lacking a balcony but will be where the voyages are. Chair included of course. The masts will only be on the main deck and the pullies for it will be on the same deck. The lower deck will be similar to the current Brig where that is where everything will be. The layout can even be much the same for simplification. The way down will be the same as on the current Brig and Galleon, just an open hole with stairs leading down. An alternate idea I had for the lower deck would be to make them into two halve decks. Similar to the current Sloop, but not sure how that would work out, make it a little harder to sink I think, but might just look weird. The rowboat spot will be just like the current Brig, behind the wheel. Speeds, turning and all that can simply be sandwiched between the new Sloop and Galleon and can be easily adjusted if need be. Crows nest would be on the back mast.
Why would you ever choose the new Sloop over the new Brig? Make the new Sloop more maneuverable, allow it to catch wind easier or turn better. Cog I think should still have the best turn and into the wind speed of the game, but Sloop should not be that far behind it and should be able to out maneuver the new Brig.
I think this fills that little niche where a 3 cannon broadside is missing, and allows the Galleon to have the proper amount of firepower it should have for a nasty broadside when fully crewed. The name changes allow for this ship to slide in with no difficulty, and allows for more varied ships in the game. Though, I am well aware them adding another playable ship will probably never happen, but still, I would be happy to see this happen.