He's quasi untouchable only in the sense the Stars don't have anyone on the roster who can do what he does. If he was traded, the Stars would be very small on the back end and would be down to very few guys who can actually defend.
Well instead of just saying "not a chance" or "he's not being moved", I'll play along.
Passed over in two drafts. Signed as an undrafted free agent out of Seattle in the WHL. And like the post above said, the epitome of a late bloomer. Dillon was easily our best defensemen last year. When looking at our D-core, that may or may not mean much, but he's the cornerstone of our D if not our team (Benn), and our (I believe should be) future captain. He's extremely physical, has some real nice wheels, and is essentially everything you'd need in a shutdown defenseman. Those 8 points in 48 games don't tell the full offensive story, as well. I think it'd be fair to say that once a game he'd pick the pick up and skate all the way up the ice. These plays didn't necessarily turn into goals, but he can create chances with his skating and IQ.
And in addition, he has some of the best flow in the game
As far as what it'd cost/his value, I agree with the post above. It'd take a 1st round pick +. I wouldn't say that's what he's actually worth, but what he's worth to Dallas. It'd take an extreme overpayment, that's for sure. Many people believed he, (and Brodin, for that matter), should've been finalists for the Calder. Unfortunately defensive defensemen/shutdown defensemen rarely if ever get noticed for trophies or awards.
Overall, he's one of the cornerstones of the Dallas Stars. We love him, and it seems like he really loves it here. He's young and just getting started. It'd take a MASSIVE overpayment to pry him away from Dallas.