I'm floating around a lot of fluff ideas on it right now but the one I'm liking the most is an elite force, similar to the GKs (but not really super psyker people). Not structured like a regular astartes chapter because they were formed post-heresey but pre-codex. Of the 70 marines that were able to escape from Isstvan III, 8 were psykers and went on to be the origin of the GKs, the other 62 were the origin for the Ghal Maraz, the Emperor's Hammer.
GKs are on the list, mostly because they have the most up to date marine codex out there. I'm really leaning towards going with the Templars, but it's been an age or two sense I've read their codex and wasn't aware of an FAQ updating them. I'm going to have to DL and read it while I'm at work (who works at work?)