@simple-tora
Ghosts: This is likely to be a thing we see on the Sea of Sorrow (the realm the Ferry of the Damned sails in), when that finally gets implemented, as is inevitable with the potential space and lore expansion that provides. There's pages of unused concept art for things within that foggy expanse.
Sirens: Likely to be part of the merfolk's way to lure unwary pirates in, it's be quite interesting to see. Don't know if the singing will be part of it though, sailing through the seas and being attacked from nowhere is quite the powerful scare for some on it's own.
Animals: Could do with some, but not on every island, or at least not commonly. Not ancient skeleton levels of rare though, just uncommon enough to not be driven off of every island by boars or what have you.
Whales: I'd hope to see these, perhaps attaching a harpoon to one would give you a helpful boost on the seas. Could even steer it if you got both in perhaps... as for that legendary massive one, that's bound to happen eventually. Be cool to see whales being hunted by megalodons on the wild as well.
Fog Illusions: This was part of one of my older suggestions, the particular part of which described segments of the Devil's Shroud breaking off of the main piece and drifting across the Sea of Thieves, at this point they'd likely dissipate a ways in due to the Shroudbreakers present in the region. They had flashes of lightning going through them, in the green of the Sea of Thieves' magic and the red of the Shroud, and within them you could hear and see a great many things which weren't there... or miss things which were there. Best be careful should you venture within at all, though glimpses of the future could be seen within, or at least according to that old drunkard in the tavern.
Military: I'm going to copy one of my posts about why this doesn't work:
The Sea of Thieves is an isolated area. No major navy or fleet can enter, for three reasons - 1: The location of the Sea of Thieves itself, and the path through the Devil's Shroud, is a secret known to few outside of it. 2: The path into the Sea of Thieves is far too small and erratic to bring formations or even large individual ships through. This is also why our galleons are so small. 3: Any large force which does find a way to attempt incursion into the Sea of Thieves is quickly driven off. This was shown when the entirety of the newly formed skeleton armada was fought off in a battle lasting for over a month straight.
Because of these reasons, the Sea of Thieves is a pirate safe haven. It has bountiful lands filled with treasure, no government to attempt to hunt down all pirates, and few threats beyond other pirates and some natural monsters, which are all but consistent. Should you fall in battle, or in any other way, the Ferryman will (not so) gladly bring you and your vessel back to the world of the living. As for gameplay, you already have roaming skeleton ships loaded with treasure, only attacking when attacked, you already have ambushing skeleton ships, chasing you down until one of you sinks or a significant obstacle hinders them, you already have fleets of enemy ships in open sea, and you already have fortress islands with vaults of treasure within. You also already have ‘escort missions’, you just have to carry the cargo yourself rather than sailing alongside another ship (should even one ship attack you, the game will not give anyone involved kind loading times given a three or more ship conflict). No need for a navy to add another form of existing gameplay, causing both to be neglected further.
AI Pirates: No. There are enough actual pirates on the Sea of Thieves, we don't need an AI trying to replace some, it wouldn't be better (in fact, the interactions would arguably be worse, as it'd not be varied).
Anyways, some of these are good, if already expected, ideas (and it's good to see others thinking of hallucinogenic fog). A couple don't work, but most do, and I hope someone at Rare sees this more than most posts out right now.