I think that Cook is on the coaching staffs bad side and could be on the way out. Or rumor would have it that way anyway.
As for Watts they may have realized he won't bulk up enough to be a power forward and have plans to use him as a mobile marking target up the ground or an intercept defender where he can use his mobility, good hands and foot skills to hid advantage.
Also as Jamar gets older I would expect Clarke may eventually have to start playing more time in the ruck as Melbourne's current back up rucks look pretty ordinary IMO.
Watt's looked really good behind the ball, not sure if he'll ever be able to play on the gorillas (would need some bulk as you say) of the comp but as a +1 in defense he looked impressive. Reads the play really well, takes a strong mark and uses the ball really well. He could actually turn into a Ted Richards type