I like what you have going. Get some small fill-in tattoos. If you want to use fillers, get stars and dots. Don't add some weird background. A "graywash" won't look good.
I have that kind of decorative work on my feet (not a mandala). I elected not to go down the sides/near the bottom because it will fade and won't last nearly as long. You'll be getting to touched up a lot if you go down the sides like that. Doesn't mean you shouldn't do it, but just go in knowing that.
Welcome. Honestly, I'm not sure that I'd opt for any kind of background. It looks like there are still a few places for some filler (bugs, flowers, abstract whatnots), but outside of stars and dots I'd probably just leave the skin open. If you have an artist in mind who you trust, his or her view is definitely worth hearing.
Hi there, I found this forum when I was searching for tattoo aftercare advice. I've got an appointment for my first tattoo this wednesday; the giant tegaderm/saniderm aftercare thread is an excellent resource. Thanks to all who posted their experience!