My legs are the same. Lots of unrelated tattoos in black and gray and color. You can definitely build a sleeve that way. I've seen lots of beautiful sleeves built like that. :-)
Any tattoos on the sides/bottom of your feet won't last and will fade constantly. Tops of feet are OK; the rest, not so much. Also, as @SStu noted, feet take forever to heal. I was basically barefoot for a month.
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.
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.
HOLY SHIZ. I'm glad you are OK. I take it off it it springs a leak. I have always worried about that kind of thing happening. :-/ Just glad you (and your leg) are here to tell the tale.