It seems all the other First Founding chapters get their own "official" successors, except the White Scars and Salamanders. I think it would have been neat to see an official Mantis Warrior build with the time line being pre-Badab War, instead of one of the Ultramarine successors, like the Doom Eagles. As for the Salamanders lexicanum says there are none known but has two suspected. The Black Dragons could have been an interesting build, giving them Gene Seed Anomaly and something like Natural Weapons. Who's with me for official White Scar and Salamander successor builds in the next book?
If this is already been discussed, I was not aware and wasn't about to hunt through the pages to check, sorry.