As someone who helps run an Island Sanctuary community of 60k+ members, one of the biggest complaints I see daily (that I also agree with) is having to gather certain materials because the workshops use them more often than other materials. Laver, Sap, Rock Salt, Copper Ore, Hemp, Cotton, and Tinsand to name the "Big 7". The Granaries cannot keep up with the demand of the workshop, if only we could do something besides having to mindlessly gather these rather poor loops the game currently has for us.
Introducing, the Exquisite Exchanger! All common (gatherable) materials currently export for 3 Islander's Cowries each (the green ones). The Exquisite Exchanger would let you spend those rather-lacking-in-use cowries on any common materials of your choice, but of course at a premium. I think ~5-10 cowries per material (around 2-3x) would be a fair price to exchange for materials.
"Wouldn't that defeat the purpose of gathering the materials?"
No, as the Island Sanctuary was created in mind with "chill, relaxing, gameplay". While some people enjoy doing the Laver or Copper loops, I rather despise doing them and would much rather do loops I enjoy such as Sugarcane or the mountain peak instead, export the mats I get for the green cowries, and turn them in for the mats I want. This would be less efficient on my end, but a much-needed Quality of Life boost for material gathering for those of us who are struggling to keep up and really hate diving into the ocean or flying on the Copper route.
You can also export materials you just have tons of (either from the Granaries or from general gathering) that you have no use for to exchange them for more of the mats that you need.
"What about when new content comes out? You'd be able to instantly get all the mats you need for all the upgrades!"
The Exquisite Exchanger will update with new materials once you "complete" that patch (e.g., once you complete the vision and finish the Tataru quest for that patch). For example, all the mats from the original 6.2 patch (Ranks 1-10) are purchasable once you finish the last vision at Rank 9. The 6.3 mats become available upon finishing the Rank 11 vision. The 6.4 mats with the last Rank 15 vision, and the upcoming 6.5 mats with the (assumed based on update patterns) Rank 17 vision. This would mean you would no longer need the materials for the content and would only need them for the Workshop at that point.
"Couldn't they just add another Granary and call it a day?"
Sure, but I think this would be a less overpowered way to let players get the materials they need, and also add a much-needed value to Islander's Cowries which I feel get you very little. The exchange rate for Materia is rather poor, leaving the only real purpose for them to be Hi-Cordials and Glamour Prisms.