hem, hrm, I think I cooked some up myself once, w/o recipe.

As you say, just use some diced boiled potatoes, peas,
add fried onions, garlic, ginger, maybe a bit of bell peppers or mild chilies.
season with "curry" powder/garam masala, maybe cumin, fennel...

problem is, real samosas are fried, which I can't do (and won't).
I used ready-made leaf pastry and painted with an egg/turmeric mix...