I think Dahlin could be a special player but I don't see how he could possibly transform the defense position?
Let's say he reaches the pinnacle of his potential - he'll be Erik Karlsson-ish offensively, while handing his own zone like Drew Doughty. That would make him, well, Raymond Bourque-ish.
He certainly isn't going to out-perform Karlsson offensively and score 100 points. And his style isn't similar to Rod Langway, where he's a stay-at-home D horse. If all goes well, he could be a better Drew Doughty, which is still future HHOF quality... but there's nothing he can physically do to "transform" the defense position.
Personally, and this is just my gut hunch, I think he ends up very similar to Seth Jones... perhaps a little more "wow" factor. But I don't expect him to be Coffey+Lidstrom rolled into one.