An own discovery: Tomato purée with like a spoon of peanut paste.
Immediately makes it creamy and tastes really nice.
Obviously, you can substitute the tomato purée for tomato paste + water. And canned, diced tomatoes are quite fun, too, as you get a fruity bite every so often.
I’m guessing, peanut butter would work, too (tomatoes taste fruity with a pinch of sugar), but I’ve never tried it.
As for spices, some pepper, chili or (balsamico) vinegar works well.
The whole Italian spices are still good, especially basil. Or alternatively, you can add various vegetables, like you’d put in a soup.