I would just like to add to the lists
HKR - Danny Addy, Mitch Clark
Huddersfield- Lee Gaskell, Dale Ferguson, Paul Clough
Not officially signed on but these are the rumours of where these Bradford players will be next year.

Leroy Cudjoe
Ken Sio

Daryl Clark has been absolutely superb this season and only wasn't last season because he needed an operation on his knee that was held off until the Winter

3 others nobody has mentioned yet who are worth a spot in SL (one from each club) :-

Junior Sau
Chris Clarkson
Nathan Mason

Cudjoe & McGilvary are top players and there's a few others at Huddersfield too.

From HKR, Clarkson can do a job but he's a workhorse backrow who's value is in his utility and work rate. He's not a captain.
Mantellato and Sio are good.

From Salford, well I'm struggling a bit but then I've rarely seen Salford this year so I'm not judging them really. Carney is obviously a dangerous player but has he got his head screwed on right?

In fairness, Clark wasn't one of the players mentioned.
Even Westerman is beginning to realise some of his early promise on a consistent basis now he is at Warrington.
Some players do go on to play as well as they did at Cas, but it does seem to be the exception rather than the rule.