He's not super valuable, but I think the same reason I'd want him is the same reason why Vancouver wouldn't be too keen on trading him for a rental. He brings a physical element that a lot of their defense is lacking. He's also close to NHL ready (according to Canucks fans) so trading him for a rental is something I'm not sure they'd do.
I'm not sure I buy the hype they're throwing out with Brannstrom, but if he is a long-term piece for them, they'll have 3 extremely undersized puck movers on their defense with Hughes, Hronek and Brannstrom. Pettersson's size and physicality is a stark contrast with the general softness of the rest of that defense.
Not only that, but their 2 best D prospects outside of Pettersson (Willander and Mynio) are more of the same: 6'-ish and 180 lb-ish puck movers. I think Mynio and a 1st for Pettersson is a lot more feasible just because of how redundant Mynio is with the Canucks, while Pettersson is a much more unique player in their system.
As an aside, I actually wonder if Mynio makes sense for the Penguins as a target. Especially if they add Jiricek, they're not exactly lacking in puck moving on the defense. That said, all 3 of Jiricek, Brunicke and Pickering are pretty big defensemen, so I think I could see the appeal of having a smaller puck mover type with them. Especially with him being a good skater that can mask some of Jiricek's skating issues, I think you could go with:
Mynio-Jiricek
Pickering-Brunicke