Not sure what you mean by 'filler' plants... filler in reference to padding out a flower display? Or a plant that's a filler in between other plants in the ground?

Dianthus , cornflower,dwarf iris, flag iris, crocosmia, hebe, fuchsia, daffodils, tulips ,Forsythia, roses, miniature roses,rosemary,lavendar. but if you go an look at my pictures yo will find some floral arragements amongst them , which will give you some ideas.
I enjoy floral arranging so i know what im talking about, and have done for 30+ years

Hebe - flowers and foliage, ivy - green or coloured, skimmia - foliage,flowers and berries, camellia - foliage and flowers, eleagnus - foliage, perrenial sweet peas, the flowers are smaller and more waxy looking but may not have the fragrance of annual sweet peas. Escallonia - foliage and flowers. Alstromeria see Erkabobbys page. The list is endless. Did you have a particular season in mind?