Option 1, New characters. The problem with having new characters is making them as interesting/awesome as the old characters. A lot of the times, this is a fluke that happens by blind luck, which is never a good thing to be told when making a gamble. Also, killing off old, beloved characters isn't always a good idea, as it can polarize fans.
Option 2, Old Characters, New Actors. The problem with keeping old characters but recasting them is they'll use the wrong actors/cast actors who look, sound and act nothing like the original actors did. It's bound to happen. Just look at the guy they cast for Kyle Reese in the upcoming Terminator flick. Just not right. Or the oracle actor switch in the third Matrix (unfortunately necessary in that case, but still quite jarring and unsuccessful).
Option 3, Old characters, Old Actors. The problem with keeping the old characters and original actors is that they're, well, old. It limits what you can do with the story provided the make-up department and digital effects crew aren't willing to make the cast younger. Which is a lot harder to do than make a young person look old. It's doable, but honestly the voice is still going to sound old (while I enjoyed the DLC for Alien: Isolation, Yaphett Koto sounds positively ancient) unless you have a voice actor do the voice effects, and then there isn't much point in having the original actor to begin with. You might as well go back to option 2.
