Thanks Fallen. I never thought about making them be created from the EC gene-seed.
I chose the Imperial Fist Gene-seed because I always like the Fluff for Rogal Dorn and the Fists. Dedicated, loyal and zealous are themes that I tried to incorporate into my chapter's background. I never really liked the Ultramarines or their backstory enough to want to make a successor from them. My army starded off as an significantly divergent chapter and I have tried to represent that in their fluff through their organization, fighting style and viewpoint.
I can see how their persuit of perfection strongly hints at an Emperor's Children successor. Unfortunately, I don't think the Adeptus Terra would sanction the founding of a new chapter from the gene-seed of a Legion that turned traitor, otherwise I might be tempted to change TMR's origins.