Groove & Atmosphere
Of course your fillers may also have parts with an awesome groove or with a great atmosphere, just like your standard hit and slow songs. The difference, however, is that the groove and the atmosphere may be broken or may even completely change over the course of the song. As mentioned before, fillers are great if you want to try different instruments that are unusual and that are not necessarily compatible with your own sound and style.
Just make sure your fillers will not be boring.
Mix up slow parts with faster and heavier ones, come up with some unexpected breaks. Maybe you could also use some exotic instruments, or use other elements from other cultures, styles, or genres.
Make sure to surprise everyone by being unconventional, as long as it fits the overall atmosphere of the album.