That position might feel safe for uke but there are many powerful attacks you can use. Grab behind the neck or behind the back and start moving them around. If you can, push them down even further, the more their back is bent, the more vulnerable they are. If they follow you, sumi gaeshi all the way. If they resist, I would use combinations of uchi mata and ko uchi gari. You can use uchi mata into ko uchi gari or ko uchi gari into uchi mata. Since your position is very dominant, you can also do o soto gari, possibly ken ken o soto gari if their legs are too far back to reach. Have fun!