The bottom end of his upside is that of an above average #1 center, but it's reasonable to believe he'll become a franchise center. He loves having the puck on his stick and seems to prefer to set up his linemates than to shoot the puck. He's able to find passing lanes and thread passes that most players simply cannot. He's at his best when playing with a finisher who knows how to find open space, like Brendan Gallagher. He also a heavy shot that should be used more often, but he needs to work on his release which could be a lot quicker.
What separates him from a center like Seguin or Nuge is his huge frame and his physical edge. He loves dishing hits and is strong on the puck. As well as being a great playmaker, he tends to play a power forward type game. He loves working along the boards and infront of the net. One of his problems is he doesn't seem to realize how physically strong he is yet as he too often relies on his stick work rather than his body in puck battles. This leads to him losing puck battles or getting offensive zone stick penalties. He's going to continue to get physically stronger and heavier and should top out at 6'1 215lbs in a couple of years.
He definitely needs to work on his defensive zone positioning as he often looks out of place or confused in his own zone. He also rushes the play when he gets the puck in his own zone which can lead to costly turnovers. Another problem is he tends to over think or over control the puck when entering the offensive zone which can lead to costly turnovers at the blue line. However, he is able to gain the zone without dumping the puck which is very promising because he, Eller, and Pacioretty are about the only forwards able to do this.
I disagree with your belief that he's having a sophomore slump. He producing at a respectable pace despite playing 3rd line minutes and on the 2nd / 3rd powerplay unit. I thought he'd have a break out season this year based on his performance early in the season. His line would lead the team in scoring chances every game and was being matched by opposing teams' top D pairings. But his line was broken up shortly after and his minutes reduced in favor of lesser veterans.
For an idea of how well Alex can thread the needle and make tape to tape passes that lead to prime scoring chances:
Game 2 of the playoff series vs Ottawa:
Game versus the Wild. His line was being matched by one of the best D pairings in the league in Brodin-Suter. He easily gets by Brodin, then saucers the puck over a sliding Suter for Gallagher:
I hope this gives you an idea of who Alex Galchenyuk is, and why the kid was chosen 3rd overall after missing his entire draft season.