First and foremost I'm going to instruct you to take this rumor with a healthy dose of skepticism given the source of the rumor.
The rumor comes from former Toronto sports writer Howard Berger, formerly of Sportsnet, and given his propensity to throw darts at the wall in recent years I question everything he reports. That being said Berger is now reporting that members of the Vancouver Canucks front office are currently mulling over the possibility of an offer sheet for Toronto Maple Leafs star forward Mitch Marner. Furthermore Berger is also reporting that the Canucks have also discussed the possibility of a trade involving Marner with the Maple Leafs, although he did not share any details regarding how far along those negotiations might be.
Again I must stress that I find this report from Berger highly dubious, especially when you consider the fact that the Vancouver Canucks have already dealt away their first round pick for 2020 as part of the J.T. Miller trade with the Lightning. That means that in the event of an offer sheet the Canucks would have to forfeit their first round draft pick in 4 consecutive years, given that you only have a 5 year window to make good on the compensation for the offer sheet.
Additionally it seems highly unlikely that the Canucks would even be able to pull off this move while remaining compliant with the National Hockey League's salary cap for the 2019 - 2020 regular season. Keep in mind that although the Canucks do currently boast $5 million in available cap space, that is with young star forward Brock Boeser still not signed to a new deal. Boeser's contract will almost certainly eat up all of that available space which leaves one to wonder how in the world they could possibly fit Mitch Marner under the cap. You also have to take note of the fact that the Leafs would be entirely unwilling to take any kind of salary dump from the Canucks given their own cap situation, so unless there is a third team involved here I don't see a world in which this could even be a possibility for either team.
The response to Berger's report has been predictly negative, and I fear he may be throwing away what is left of his reputation for a little attention in the middle of the offseason.