The probably with that is, when continuing on from existing material, whether the tone is different or not, there's still going to be restrictions on what you can and can't do*. Whereas starting fresh enables you to have complete freedom with the material. Now that's not to say that they couldn't start fresh and still use Bana, hell, he did a decent enough job in the first movie, but I would rather someone new, because although he was decent, his performance wasn't anything amazing or particularly memorable. I like the idea of starting fresh with a new face.
*Think about Batman Begins, for example, if it was a continuation or even prequel of the original series Nolan would have to adhere to the limitations set by the previous films: Joker would have to be the Waynes' killer, as well as the general origin for the characters would have to be the same, he wouldn't be able to use some of the characters that were supposed to have died, in fact pretty much everything about the movie would be different