This is what I think.
Forbidden Valley – one new family flat, and one new thrill flat in place of Ripsaw. At the moment there is a serious lack of things that are suitable for younger children in this area of the park, and means a lot of back and forth walking for groups / families with childeren of different ages. A flat ride replacement for Ripsaw, as well as a new family flat ride, possibly given a space theme to tie in with Galactica.
Dark Forest – one new thrill flat. I think there is room for a flat, and it'd be highly beneficial for the land, absorbing the large crowds that the area attracts.
Cloud Cuckoo Land – one new family flat, in addition to the land's retheme. The new flat ride would be to cater for the lack of family rides in the Dark Forest, and as CCL isn't a long walk away, should mean that there isn't any problems with lack of thrill or family rides in each area.
X-Sector – one new family flat. At the moment, there is just nothing for younger people to do in the area, as Enterprise will often been seen as too scary, even if children are tall enough to ride it.