Just plants, whatever you like. The deeper corner sections provide room for taller plants if you want them. If the griselina is a hedge I would probably leave it, depending on what's on the other side, and put cottagey flowers in front of it.
How acidic is the soil? Most plants aren't fussy about a bit of acidity, it's the other way round that's a problem - ericaceous plants won't be happy in alkaline soil.
A garden is an oasis for creation, available to anyone with a little space and the compunction to get their hands dirty.