IF AppleGrant are in charge, I think we're virtually guaranteed a good product. We've seen the damage done to the series when left in hands other than theirs already, and the things that make the books work (the compelling characters, the presentation of deep and important concepts in an organic and simple yet not dumbed-down manner appropriate for younger readers, the constant balance between sheer escapism and GrimDark Angst) need to be retained. I believe that the original team has the best chance of keeping these qualities in tact, but theoretically a sufficiently talented fan who cares about the series while still possessing the objectivity and personal creativity necessary to make the enterprise worthwhile.
While a sequel might be interesting, I don't know how possible it would be to retain the elements that give the series its soul given that most of the characters are dead and the secret is out. The best sequel I can imagine would be a one-off "Human Chronicles" sort of idea.
I do think a straight rewrite of the series by Applegate could be a very good idea. It could serve to remove most of the filler, compensate for the deficiencies of the Ghostwriters (the quality of whom vary! I thought The Deception by Elise Smith was fantastic and worthy of the series, whereas The Return by Kimberly Morriss took an interesting plot and killed it with Cheesy bold-lettered purple prose!) and even clean up the writing in a few of K.A.'s earlier books (there's a clear shift in quality going back from the fantastic last two books to the surprisingly dumbed-down first one). More importantly, it'll keep the series in the minds of a new generation, something that I think will genuinely benefit the readers themselves (even if it does mean we get a bunch of tweens on RAF)