I think Boston makes a competitive offer if Krug walks.
I think Edmonton would have a better chance than some would like to believe. Larsson and Russel are both RD and will be UFA in the summer of 2021. Larsson has no trade protection and Russell has to submit a list of 15 teams he would accept a trade to. If they landed Petro, they could flip one of them for a futures asset or cheap middle 6 forward to help with their depth. They should be a clear playoff team if you replace Russell with Petro next year and they have enough cap flexibility from 2021-2025 that you can really sell the ability to build a great core around McDavid, Drai, and Petro.
This one is less likely, but I could see the Devils doing what the Rangers did last summer with Panarin. They have three 1st rounders this year and zero cap concerns for the next couple years. Subban and Schneider both come off the books before they owe their current young talent raises (Hughes, Zacha, Butcher and Blackwood if he is truly the guy). They could offer Petro $12M AAV by 7 years, still be able acquire a couple other good veterans this summer, and not have that AAV hurt them for the first 4 years of the contract. I'm not sure Petro wants to be a part of a rebuild, but that would be a hell of a lot more than anyone else is offering and they would be a bubble team or better in year 1.