I would love to see Sandman in the Frightful Four, and you would think Marvel could make that happen.
If not, either use Quicksand - a female version of Sandman who was also a member of the evil FF at one time, or keep a male sandy villain and call him something else - Sandstorm, Quicksand, Terror Ferma - what ever!
I do think it's best to keep the sand element rather than using Hydro Man, Thundra, etc. - keep it as close to the original action as possible, and issues #41-43 would make for a Fantastic sequel! This is why I do not want to see Puppet Master in a sequel, THE best story where the Thing was turned against the FF was the Frightful Four one!