Black Templars, the Salamanders, or the Imperial Fists would all make sense since they are three iconic chapters.
Raptors. There are already 2 close combat chapters (Space Wolves, Blood Angels) and 2 flexible chapters (Ultramarines, Dark Angels). It would be nice to see a lesser known chapter that focuses on ranged combat with interesting customization options. The Raptors actually utilize camouflage.