From my POV, a hockey player is far from a finished product at 18. The game is so complex, skating in hockey is enough to be a sport in it self, then you have the physical aspects AND the whole stick/puck part. These kids starts to do the work on their end result when they are around 14-15, and most only get to the level they can get around 22-23, if not later. EVERY year in that span, at least 8, is each as important.
I remember Karlsson as a 17-18 y/o, he wasn't really close to were Dahlin is. He was so thin and small, albeit tremendously cocky with the puck. But even if he wasn't really a regular early at 18 for Frolunda, he was like 155 lbs it's felt like, I remember a preseason game between Ottawa and Frolunda in Gothenburg in 08' (Ottawa started the season in Europe). Karlsson got little ice time at first, but then just grew and grew and grew during the game. At the end he wasn't almost putting up a show. Chris Neil was half for fun mocking up a player on Frolunda. Then a few shifts later Neil got the puck between the redline and offensive blueline and Karlsson landed a Kronwall hit at him at like 180mph. Neil didn't fall down but lost his breath of the hit and had to go to the bench. That's Karlsson in a nutshell, there is nothing he can't do. Karlsson was of course drafted by Ottawa at that point and I remember the reactions on their bench after that hit, it was like wow we got this kid? He was so small and weak but he is amazing. Karlsson just took it and ran with it. Became so much better every year till he got where he is right now. Rose to every challenge ever put infront of him.
Rasmus Dahlin is a completely different type, at 16 y/o he can look like he is on a different level than everyone else -- in some cases -- in the SHL. All of a sudden he isn't skating 8s around everyone. On the back check in a situation everyone is scrambling to get back, Dahlin all of a sudden notice that he skated way to fast and have to literary wait for the forwards to catch up before he can hit them.
He is the best Swede ever at 16 y/o, and it's just not even close. I didn't see Sundin at 16, but I saw him at 17-18 and Dahlin is better. How good is Karlsson really? I would say that he is top 10 in the game easily, probably up towards top 5. That per definition would make him a very very good first overal pick. Will Dahlin become better than that? He would of course have a very long way to go. But he has that potential -- and probably then some. He is the best D prospect at 16 I've ever seen to be honest. We haven't had a real generational talent at D in a long while. Not McDavid/Crosby/AO level. Dahlin must keep taking huge steps -- but he COULD be at that level in 12 months.