Boston finds itself in an interesting spot at the moment. Its core is aging and centered around Bergeron, Marchand and Krejci. They have all but their 7th round draft pick. The Bruins wouldn't be totally stupid for going after Nylander, or at least speaking to him. He can play center, and would help the Bruins retool and remain Cup contenders for the next half decade. Hear me out here. Bruins sign Nylander to 6X6.3 million. This would put them over the cap. However, they do have several aging centers in Krejci and Backes, who between them take up over 13 million against the cap. Whats a team that is desperate for centers? Montreal. They have a ton of cap space and should be convinced to take at least one of Krejci/Backes for cheap. Say krejci for a 2nd with no salary retained, 2nd and 3rd if they retain about a million. Backes they can have for a 3rd with a million retained. This is all theoretical, but it could work. This would help the Bruins also resign some of the young guys. Contract breakdown after this year would look something like this
Nylander 6X6.3
McAvoy 2X4 Bridge or 8X7 long term
Carlo 2X2.5 Bridge or 6X4.8 long term
Donato 2X3 Bridge
Hienen 3X3 Bridge

This is allllllll speculation and me throwing out what I would do. It wouldn't surprise me if one of the Bruins rookies was traded, but it would be a rather short-sited play. Is this scenario will almost certainly never happen, just a fun little hypothetical.

Leafs would love to sign Nylander for the deal you suggested and would.
Backes and Krejci have restrictive NTCs, may really difficult to trade them

Nylander wouldn't sign an offer sheet, especially not for that price.

MTL sure as hell won’t help BOS for cheap. Backes zero chance they would take him on unless they get heavily compensated. Krejci on the other hand I could see MTL having a little interest in if all it would cost is a 2nd. Plekanec would have to go (either in this deal or in a separate one) Only reason I say this is that MTL for sure next year (or two) will want 2-3 C positions freed up for guys like Kotkaniemi, Poehling, and Suzuki. They can currently use Pleky as a mentor, but Krejci isn’t who he used to be. Not sure if that upgrade is worth taking on the last few years of his deal. Not impossible though.