They included too much, too soon into chronicles of riddick. that film itself could and probably should have been stretched out into an entire trilogy, or at least the first 2 parts of it. The first film could have been Riddick on the run across the galaxy with Toombs on his trail. The necromonger stuff could have been slowly introduced and worked as a sort of background mystery throughout, not really coming to much plotwise. Just lingering there in the background.
The second film could have been set on crematoria and things start to get interesting from here regarding the necromongers. While riddick was on crematoria the necromongers could have taken helion prime. The final film could have then moved onto the necromongers and furya fully, with an all out war erupting, ending with Riddick taking down Colm Feore. This way the first two films could have been kept similar in tone to pitch black, slowly changing into the big epic finale.
As it stands theres a huge jump between pitch black and COR. It could have been done gradually. I commend them for trying because they really where going for a sort of adult star wars kinda thing. Creating scenarios, enemies, worlds, races etc, but they did it too fast. I still love the film but the ending of Riddick should have been the ending of the trilogy. Leave it on that awesome cliff hanger of "you keep what you kill" for ever.