I posted previously in the wrong thread. If anyone has ever played Pirates of the Spanish Main for NES, this is sort of what I envisioned Adventure Mode being like.
You start out as a person with a ship somewhere in the Caribbean. You gather a crew, then either purchase goods to sell somewhere the price is higher, or go pirate other ships. Of course, there's the Dutch, French, Spaniards, and English all in the waters. Local lords will offer you Letters of Marque--basically, an agreement that if you pirate the ships of the people they're at war with (which changes), they'll give you land and money.
You can also assault and sack towns, once you have a fleet of enough ships. Disarm their guns, destroy their barracks, and the loot is yours. Of course, expect the Navy of the owning country to send ships after you as a pirate. EDIT: It's a merit-based system; countries you work for (or at least whose enemies you work against) grant you influence, wealth, and military power. Those you fight against try to kill you. It also keeps everyone from saying 'I WANNA BE A PIRATE' and just doing their own thing (unless they're good enough to justify it), which sort of destroys the history and ambiance of the world Muse has worked to create. It also means the PE No. 1 in the world would be damned good at what they do.
The ability to use multiple ships in your fleet is limited by the total crew (whom you have to pay, or else they'll mutiny), and the cargo you carry is limited by weight per ship. Think that a Squid wouldn't carry a lot of cargo, but would be good for carrying small and valuable items.
I really would like to see some air-to-ground combat; cities defended by heavy and light weapons, troops battling on the ground. Imagine a giant dirigible dropping stacks of bombs (a la Command & Conquer: Red Alert 2) on enemy troops and gun emplacements. Right now we just face off against other airships, but it would be very empowering for the player to see just how much destruction a Hwacha volley could wreak on enemy forces on the ground. It would also bring a welcome level of complexity to PvE where enemy pilot AI would have a tough time keeping up with the talents of the playerbase.
And along with Ofiach, I would like to see a giant Koopa Kruiser-style ship of chained together airships that a large PvE battle is waged against. Think Independence Day.