Well I wouldn't complain if Orlando Bloom played Ishida that's for sure, lol. I'm a big fan of his and would watch if only for the prettyness factor, but I also think it would be an awesome role for him
Hayden as Ichigo would be fun too, and yes I agree with a previous poster that he's a decent actor as I've seen other work of his besides Star Wars. It's not his fault George Lucas sucks at dialogue...
For Aizen I have two suggestions, depending on if we're talking strictly a young cast or all over the place...
Cillian Murphy would make an excellent Aizen if we're going younger (he played Scarecrow in Batman Begins and was excellent in Red Eye) cause he looks wonderful in glasses (truth be told I wouldn't mind him as Ishida either) and is incredibly talented...
But my true dream Aizen? Alan Rickman....I know he's a bit more...erm, mature (I hate saying old cause Alan looks incredible for his age...I can't believe he's 60) but he's fantastic at playing evil, looks great in glasses and has that voice...he could read a phone book and I'd be mesmerized...
Anyway...chances of a live action bleach, at least a Hollywood version are pretty slim and they'd probably ruin the story, but hey, it's fun to play around with casting choices...