Quoting: dannibalcorpse
I think the point that he seemingly wants out of Montreal makes his 15 team NTC more like an 8-9 team one - he'll submit 15, of course, but I'm sure there are a few teams that would be on the list that he would waive for anyway. In the spirit of your question, though, I'll try and map out what teams would realistically have mutual interest in a Petry trade.
The obvious teams that we all assume he doesn't want to go to: Ottawa, Toronto, Winnipeg, Edmonton, Calgary, Arizona, Buffalo, Vancouver
Teams that aren't contenders now and wouldn't swing a deal for a mid-30s D with his hit: Seattle, San Jose, Anaheim, Chicago, Columbus, Detroit
Teams that are basically set at RD and don't need/can't afford Petry: St. Louis, Colorado, LA, Boston, NYI, NYR, NJD, Vegas, Tampa Bay, Washington, Minnesota
That leaves Nashville, Philadelphia, Pittsburgh, Carolina, Florida, & Dallas as the remaining spots that *might* take a swing at him.
Nashville has Dante Fabbro, Alex Carrier, & Philippe Myers as their RHD under contract next season, and has $27M in space - so even if they re-sign Forsberg, they can still be considered in play.
Philadelphia is a wild card. Risto is their only right-shot D with significant NHL experience signed past this year, but are they tearing it down now? Petry would work if they want to fast re-tool, not if they're going full rebuild.
Pittsburgh only has John Marino & Chad Ruhwedel signed next year. If Letang surprises us all and leaves the Pens, Petry could become a target for them.
Carolina's more of a longshot - Ethan Bear and Tony DeAngelo are both RFAs, so likely back next season, but still a possible place for Petry.
Florida is the same - they have 3 right shot D on the roster for next season, but both Brandon Montour & Mackenzie Weegar can play the left side, so they have flexibility to take a swing at Petry in their top 4.
Dallas doesn't have a ton of cap room, but might have a John Klingberg-sized hole in their lineup if he walks. I think Klingberg will sign for more than what Petry makes, so you could consider them a possibility.
Not a really enticing list, when you get down to it. There's a couple of teams on that third list that I could see being wild cards as possible destinations, but the ones who have the lineup spot for Petry are also the ones who would need to move cap out to fit him in - and I'm assuming Montreal isn't going to want to take a ton of cap back either.
I think MTL would be open to retain salary or take a contract back, shorter than 3 years. Any team not asking MTL to take a bad contract, or retain salary, could have Petry for a cheap price.
I always thought Nashville was a good fit, as RHD seems to be one of their weakness. He would fit well with one of Josi or Ekholm. Since they have cap space, i think Habs don't retain on Petry. But the thing is, they traded Ryan Ellis last summer, not sure why they would trade for Petry.
I agree Pens and Stars would have interest if they lose their own RHD to free agency (Letang and Klinkberg). Then Habs wouldn't have to retain salary, since they'd have cap space with those departure.
MTL would definitly have to retain salary for a trade with Florida, but i could see them swing for Petry, try to keep their Stanley cup window open for a couple more years.
Philly should rebuild, but they never will. But i'm not sure they trade for Petry, they have Risto and Ellis signed long term on RHD.
I could see Buffalo and Columbus as potential destinations, if the price is reasonnable. MTL wouldn't have to retain or take cap back. While they're not contenders, they both could add a top 4 RHD. If Petry wants to be close to Detroit, and doesn't care about winning in the next couple years, both teams makes sense. He could then be traded on his last year of contract to a contender, as a rental.
Like you said, a couple more teams could be interested if Habs retain half Petry's salary. A 40+ points RHD vet, on a retained 3,125M for the next 3 years, could draw interest from contender teams.