The Randy Orton diary was by no means a bad one; he was using Orton right as his character was bordering on his face turn in real life, which meant there was some good characterisation in there, mainly in his thought processes - his opinion of Triple H, how he felt he should react to guys like Cena or Sheamus... it had potential. Plus, on the storyline front, he had Vince call Orton to his office and tell him that he'd be mentoring an NXT Rookie - Orton didn't want to do this, setting up an angle we never got to see, but the possibilities were many from that one 'chapter' alone. There are plenty of ways to not
limit oneself, foreshadowing future feuds, setting up conflicts with more than one other superstar, authority/diva angles etc.
In terms of diaries in general, I'd like to see more of them. The potential to get to grips with one particular character is the main thing for me, considering you can go even further into their psyche than you can just booking normally. The one problem I'd personally have with it - I'd take a rookie and take them to the World Title in, say, a year - would be I'd constantly be thinking about their next feud, which would mean I'd think about how to set up his opponent
as a credible threat, meaning I'd get caught up in that guy's feuds and then I'd be wishing I could book the whole show... and voila, I'm just writing another BTB. But yeah, that's not to say the concept wouldn't interest me. Something like a Dean Ambrose one, where the gimmick is someone clinically insane, could work, though you could argue the interest in those characters is built from the mystery behind them. I'd read one if there was a decent one around. Or any one around.
Book what you want, write how you want, do whatever you are going to enjoy