He's "Mercurial Pete". He has all the talent in the world to be a dominating, dynamic, gamebreaking player when he feels like it. He's entirely capable of playing an engaged game where he doesn't really "bully" anybody physically, but when he's dialed in, he becomes extremely difficult to take the puck off of. He can stickhandle in a phonebooth with the best of 'em, he's got an elite shot and he's a deft passer with terrific vision and creativity. That and his reach all allows him to be one of those "slippery" players who can play in traffic and tight areas and be extremely difficult to get a bead on, and he honestly has a bit of a chippy competitive streak to him when it's activated.
Problem is...way too much of the time, he just doesn't seem that interested in being as good as he possibly can be. He doesn't have that lunatic "drive" that guys like Croz, MacKinnon, Draisaitl, Hugheses et al have. He's far more of a "hobbyist" with insane talent.
He's also just physically not built for it. He's tall, but he's so spindly and weak, and the skating is "fine" but he's never been a strong bulwark on his feet and his stride is super inefficient and shaky. He's not "explosive" or "dynamic" physically in any way. He's not the same as the Sedins obviously, but where he wins is similar...in that it's all cerebral and in cleverness and clowning on guys in short areas.
The other problem is...unlike the Sedins who obviously had at least one other linemate dialed in to their frequency...Pettersson has spent a lot of time, especially more recently, saddled with guys who aren't on the same wavelength at all. Sprong/Heinen/Garland...these aren't "top line" caliber players with vision and ability to function and read plays at his level. DeBrusk is...a work in progress trying to find that mesh. And all of these are far better than most of what he's spent time with over the last couple years (aside from a hot streak with Kuzmenko).
It's a bit of same issue that Team Canada ran into with Crosby over the years, where it takes a certain type of player to "fit" with this type of player (and Petey isn't Croz, and doesn't have the same skating and powerful dynamic dominance individually either).
I think it largely is what it is at this point. I don't expect Pettersson to really "reinvent" what he is as a player. But even when he's just sort of coasting through things, he's still a PPG Center who is reliable both ways and does elevate players around him. He's just a bit of a frustrating player because he clearly has the ability to be so much better than he often is.