I have used fennel as filler with all sorts of flowers and have come up with some really fun arrangements!
Just about any herb you grow can be used as filler to increase the impact of a few garden flowers in a vase.
At certain times of the year we just don’t have quite enough flowers. This is when "filler" is really important.
Sage and lavender are used extensively in perennial borders but there is another, hyssop, that deserves more attention than it currently receives.