You dont say what sort of soil it is - is it acid or alkaline, clay, sand, chalk?
Many herbaceous perennials dont mind shallow soil and a bit of light shade, nor do many ornamental grasses. Maybe you could combine perennials and grasses with gravel? That would look nice.
i think though if you can some soil improvement will be helpfull, depending on the soil type, and will increase the range of plants you could grow. if nothing else it would bulk up the soil and increase its depth.
Do some research in books or the internet on plants for specific sites and soils, that will give you a better idea of what you could grow.
Hope this helps