The knitters already invented schlippers.
http://www.kriskrafter.com/shfeslsueapa.html
https://www.youtube.com/watch?v=pBqRDRCCKQc
As for slippers that you can wear like shoes. I gave my mom a pair of
Uggs slippers that she wears out into the world like shoes. (mostly because she forgets to change shoes). I got her the ones that have soles to hopefully reduce the chance that she'd slip while wearing them.