Hi Millie,
Slopes take a little bit more ingenuity than level areas, but a creative design can still be achieved.
There are a few things I don't know from your information, such as how large the sloped area is, if it is shady or sunny, and actually how it relates to the other areas.
That being said, there are a few general things you might consider.
Since you already have river rock and boulders in the other areas, you might consider repeating these elements. Don't overdo it. If you have quite a few, perhaps just one beautiful rock might help to integrate the areas.
You could also consider a low, natural stone, retaining wall. Can you add some lawn on the slope to break up all the plantings? I would also repeat some of the plants you have already used for a cohesive design. A rock garden?
Perhaps add some groundcover for ease of maintenance in some places. If you level out a small area, would that create a nice sitting area from which to view the fountain? How about a perennial garden?
As you can see, there are lots of options. You mentioned you have an area to the left of the walk which is wet and you have therefore planted there. If that works for you, fine. If it continues to be wet, you might consider adding some drainage pipe to take the water away.
