Its probably a little late to comment on this but, as a big fan of kha'zix I can say a few things about that could make him a little more viable. All he needs is a little push.
First off for his W, i believe a heal scale ap is total bs, if riven can have a 100% ad scaling shield i think kha'zix can get like 20% ad scale on his heal.
Second, for the isolation, the biggest problem is minions and towers, considering 3/5 of the map is filled with minions this makes it more challenging, as well as ever since season 4 minions followed people in to the river and jungle area this doesn't help kha'zix hunt in his own domain.
Third, just thought of this, maybe make the unseen threat bonus apply to his q as well. Also since slows don't stack anymore, his passive and w don't need them both. Maybe make a short(.5sec or lower, don't want him too op) snare on w? or maybe some sort of other cc.