On the one hand, I have long held that KT isn't completely making the story up as he goes, that he has always had an organizing principle, that principle being, essentially, Ichigo's path through 'reincarnation' in six worlds (okay, probably only five) and on to eventual enlightenment. We've had the "introduce all these people arc", we've had the asura arc in Soul Society, we've had the hungry ghost arc in Hueca Mundo, we've had the human arc with the Fullbringers, and so far all of those things have been relatively well integrated. Some parts were more clearly off-the-cuff and unbalanced; in particular, I think KT couldn't quite decide for a while whether he wanted Sado or Orihime to be more associated with the hungry ghosts, and when he went with Orihime that left Sado with a bit of re-orientation needed. He didn't really put in as much narrative work as that re-orientation required, though, leaving Sado peripheral to what should have been the arc centered on him as much as on Ichigo. That was annoying.
I don't fear that will happen to Ishida; his centrality to the I'm-guessing-hell/Quincy arc has been signalled from the start. The part I think is really falling apart is the convincing integration of Ichigo's powers.
( Spoilers for recent manga chapters )