I'm gonna define "uncommon" to mean, "uncommon in Anglo countries" because some of these things are very common in very populous countries but not well-known in Anglo countries.

Black salt tastes eggy and can be added to any vegan dish to make it have some egg taste. It is pretty common in India, particularly as a component of chata masala. Chaat masala is also great, highly recommend it.

Lee kum kee mushroom boullion powder (or similar brand). It is basically a mushroomy msg + salt powder and it will instantly give a savory dish a flavor boost.

Hing, aka asafoetida, provides a garlicky flavor and a strong smell. It is also a component of chaat masala. It should be kept in an airtight jar if you want to avoid your whole place smelling like it. Also avoid if you are pregnant, as it may contribute to miscarriages. The plant it is from is a relative of that one plant that Romans used to eat as birth control and I bet these are related facts.

Real chili powder. Like the only ingredient is chilis. When Americans buy chili powder they are usually getting a mix of things. Real chili powder let's you add just a nice chili flavor and nothing else.

Citric acid. It adds instant tang and is a great pantry backup for lemon juice. You can also use it to make a nice vegan ranch in combination with a vegan mayo. You can also make sour candy out of any other candy by just rolling it in some citric acid. Example: sour patch kids are just Swedish fish rolled in citric acid.

Also I'll scone sumac and green mango powder as nice uncommon spices.