In the interview with Dorky Diggity Dave, the Devs basically said that implementing new masteries would be complicated and they gave an indication that they're not devoting resources to developing new ones. It's a shame though. It would be great to add in new ones with the new abilities that have made it into the game since the last mastery update (Deep Wounds, Assassin, Unphased, Despair...years ago). For the Offensive Tree, they could expand on Extended Fury and Enhanced Fury, You could add in Extended/Enhanced Cold Snap, Shock, Incinerate, Poison. These would let people emphasize certain champs they really like and these simple additions couldn't possibly be too complicated to implement.