This is kind of how it should be, as long as the title is up for grabs then there shouldn't be an enforced opponent - unless maybe you've gone too long without taking on the #1 contender, and even then the fight should have to coincide with the champs schedule. If they're doing it like this though they shouldn't be enforcing any mandatory fights, for example - going after Cotto for the GGG fight is great and all, the fans want to see it . . but at the same time it's like their only motivation is to build him up.
Khan should have to go against Brook first and get that IBF title, winner of that should get Thurman. Honestly I have no faith in Khan, I believe if you put him in the ring with 10 contenders you'll see him lose at least 5 times. Danny Garcia outclassed him and knocked him out, if Danny can prove himself at 147 why wouldn't he be the contender?
It's only a matter of time before Khan is proven to be a pretender, if his management doesn't pick his fights well he could be journeyman status within the next 2 years.