The Sinnoh League, eh?
Let's see. How many important Trainers are competing...
Ash, Paul, Barry, Nando, Conway and... that Trainer with a Legendary Pokémon or whatever...(I'm still guessing Buck just because he's awesome like that!)
Of these trainers, the least "important" ones are obviously Nando and Conway, so I'm expecting those to be the first to lose. Maybe Nando even loses TO Conway, to show off his new team and power!
I'm quite sure Ash'll battle Conway and Barry/Paul. The reason I don't think he'll be battling all three is because Barry has been wanting to battle Paul since the day we met him, and a battle between the two would really be quite spectacular.
Now, for the Legendary Pokémon Trainer, I guess he/she is the last in line, as many others have guessed before me.
Or maybe he/she isn't. Maybe they'll put him in the semifinals, with Paul or Barry being the opponent of the final round. I think I'd prefer that, yes. Especially if they surprised everybody by letting Barry win over Paul, delaying the final battle between Ash and Paul to... after the League, or something.
As for team changes throughout the League, I'm guessing Buizel evolves. Maybe Gible will evolve once, too. Two times in such a short time shouldn't happen. It mastering Draco Meteor will most likely happen, at least.
...And that's all the predictions I've done as of yet. I'll post more once I... predict more. x3