I honestly believe Marvel has been planning a Nova franchise from the beginning. Now, it can be argued that it could have been handled better or done differently but I think the plan was a franchise all along.
Given the success of Green Lantern and its franchise of titles, movies, tv appearances, etc., it stands to reason that Marvel would want to duplicate that with something of their own. And they knew they had it in Nova.
In my mind (warped as it may be), the plan went/goes something like this:
* Take Rich off the board. Otherwise a new Nova would not be accepted by new or old readers because we'd be reading Rich's title and wouldn't give a second Nova a look. At least DnA were able to give Rich a fantastic send off. A true hero's farewell. This is a HUGE gamble because the old readers may not give the new Nova a chance and not come back when the franchise plan is revealed with Rich's return.
* Aim the new Nova at a younger demographic than Rich's. Give new readers a new Nova one that's not tied to Rich's incredible legacy so tightly that the new readers feel lost and don't have a character of their own to follow. The only misfire in this has been Sam's school life is way too close to Rich Rider's (Mike Burley - Karl Moffet, Carrie - Ginger Jaye, etc.). His home life has been different enough from Rich's to make Sam stand on his own, though.
* Give the new Nova something different than just being a new Nova. Otherwise, it's just a reboot of Rich. The back story of the Black Novas, Sam not being Rich's successor but his father's, along with the powers coming from the helmet and not from his body has made Sam an interesting Nova (or Black Nova) so far.
* Give the new Nova enough time to develop his own fan base to sustain his title when the franchise is launched. It's been over a year and a half and the sales are slipping but not free falling at this point. It's somewhat steady right now.
* Return Rich Rider to a lot of fanfare and a new title! Rich will be in two issues of Guardians of the Galaxy (#18 & #19) and written by a big name writer. So it will be noticed by fans of comics. It is also coming at the perfect time as the GotG movie will be in theaters when #18 comes out. The movie also just happens to feature the Nova Corps, Rhomann Dey and Xandar! So interest in Nova from the general public will be heightened. And most of them will remember Nova as Rich Rider from when they were younger.
* Given Rich Rider a title (Nova Prime) and make it different than Sam's. Sam's book could be more Earth-centered with occasional space stories and Rich's book could be more space-centered with occasional returns to Earth. Sam's could still be targeted for young readers (and viewers of the Ultimate Spider-Man animated series) and Rich's could be more for the adult readers with harder sci-fi elements.
* Allow for crossovers with the two titles with Rich as a mentor to Sam but with Rich still being able to learn a thing or two from Sam. Rich could be a great teacher and maybe even over-protective of Sam as he sees a lot of similarities in Sam from when he was that age. Sam could be a good student as he knows of Rich's legacy and wants to be better. Sam also might even be too eager to prove himself to Rich which leads to situations.
Anyway, that's what I think Marvel has in mind. I hope I'm right! :)