If we're talking about the absolutely superb Frank Miller/Walt Simonson comic, there's a surprising amount for them to go at. For starters they could (and I guess probably will) do all the figures in a similar style to their comic TMNT figures with flat colours and cell shading. ED-209, the T-800, a variety of weird and wonderful designs like animal terminators, future resistance, a Robocop/Terminator hybrid from the end of the story that looked pretty cool, and more besides all show up over the course of the story. It even goes as far as showing a possible future where Skynet wins and creates Terminator spaceships to conquer the universe with. I'd love it if they did new sculpts for Robocop and the T-800 to match Simonson's super-dynamic art but I can't see that happening, really.