RTD revealed his "big old plan" about Caecilius and Frobisher on the featurette for the Children of Earth
DVD. It was just that Frobisher was a descendent of Caecilius, and the murder/sucide was the timeline finally "correcting" the Doctor's rescue of the family from the fixed point of their destruction. God knows how he and Moffat are going to work the new Doctor into that. I can't say I see the point of any of it, but it'll probably be a throwaway line like Martha's cousin, so who cares? Maybe they can work the soothsayer and Amy Pond into it as well.