I enjoy the existing guilds and I think they epitomize the core experience for SoT, but I know I'm not the first to say they need some more variety and fleshing out. I think there are some realistic ways to expand on them. A lot of these ideas I know are not uniquely mine, but I think the more it is said the higher chance Rare will notice.
Gold Hoarders:
One idea for Gold Hoarders would be to have sketches of the buried locations like what you'd see in Assassin's Creed Black Flag. The sketch would show a landmark with an X (or multiple X's) somewhere nearby and the name of the island placed somewhere on the page.
*on a side note, I think the Riddles quests should have a higher likelihood of finding a more valuable chest since there isn't a chance of finding multiple chests.
Order of Souls:
There are a couple options for OoS I think could work. One new variety would be a way to "summon" the Skeleton Captains. It could work like a Riddle quest where it directs you to a painting/landmark and makes you do an action (maybe even a sacrifice) to summon the skeletons.
Another variety would be to stop a "Skeleton Ritual". The skeletons would be found at a landmark performing the ritual and if you don't stop them in time they'd receive some kind of buff. Like a Riddle quest, I think there would just be one Skeleton Captain who's skull you'd receive, but there'd be a higher chance of it being a more valuable skull.
Merchant Alliance:
A new variety for a MA quest would be to recover loot from a failed voyage. For instance, you'd be given the name of the island the ship was last seen at, along with a list of the items. When you arrive at the island the sunken ship would be found somewhere nearby. The loot you're tasked to find would either be on the sunken ship, somewhere nearby in the water, or on the shores of the island.
Crossover Quests:
I'll admit this is far-fetched and unlikely, but there's potential for a quest to sort of cross over to the other guilds. For instance, a GH Riddle quest would have you dig up a grave for additional info, and in the process you dig up a skull. Another option would be for a OoS quest where you have to capture an animal to summon a Skeleton Captain that you can keep to cash in at the MA.
What do you think? I don't think the game needs new guilds as much as they need greater variety to the existing guilds. How do you think they could be expanded on?