In my humble opinion, you can use almost any stitch as a fill... It doesn't need to be a satin stitch every time, right-e-o? And each stitch you choose gives a different texture or feel to the work.
Of course, French knots are my favorite of fills (hence the name... and the millions of French knots I've made).
Here are just a few examples of fills that are in fact not a satin stitch:
I thought I had made a lot of French knots... but check out the work done by Yuko Uemura.
I'm guessing that the green wings and face on this vintage embroidered owl from Ebbie Mae are stem stitch. and the rest would be satin stitch. Let me know if you have another guess on that.
On this pattern (pdf on my website) I've used backstitch in the chandleir and the legs and seat of the chair and bullion roses and French knots in on the chair back...
On this embroidery I've used chain stitch to fill parts of the dresses...
And here (the applique/embroidery version of the doll above from my Soirree quilt)... my old stand by again, French knots on the bottom of the dress.
I'd love it if you'd add your photos to the We Love French Knots Flickr Group so we can see more great examples of fills. Show us what you've got! I'll add them to this post.