Why not just plant it up? With the slopes it will have different micro-climates/aspects. I love a bit of elevation difference in a garden. In the Wildside garden (haven't visited it alas, but read about it), before planting, it was landscaped with mounds, ridges, and mini-valleys. I have a mini-mound + valley in my garden, and it is one of my favourite spots. It brings a dynamic aspect to any garden.
In fact, I'd be tempted to excavate some more in one or a few places, and introduce additional levels, slopes, and terraces for the purpose of planting, with a meandering path.
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