The 2007 movie, but I doubt that would be a big seller. Movie adjacent stuff like The Next Mutation is probably the most logical, but again, probably wouldn't be a big mover of merch. That would have made the most sense as a convention five-pack where they could have probably charged a bit more than retail and had a decent enough sell-through.
Really, the only "big" item outstanding is Turco. Maybe a movie 2 and 3 two-pack. A movie 3 two-pack of Casey and Whit if they still have likeness rights. Neither likely sells big though so it's probably not a priority. And it feels like if Turco was a possibility they would have done it by now.