Bo for Pageau + prospect may make a bit of sense for both teams. Both are excellent at PK and faceoff with Bo highly likely not resigned in Vancouver. Bo is too good to be a 3rd line center and he can play on Isles's 2nd line by moving Barzal to Nelson's wing to create a much improved top line. I am both a Canucks and Isles fan and watch both teams a lot and think this may benefit both teams. Canucks can use that money they would have to resign Bo to go after an RHD from another team in his prime or a UFA this coming summer.
Mayfield likely will not be retained easily but Isles is right now capable of contending for a playoff spot so he will not be traded until near the deadline depending on if Isles can make the playoff or not. He is highly valued on the Isles team, a well-liked teammate who can handle the rough stuff, real good in front of the net boxing out opposing players, and is a big part of why their PK is top 10.
I recalled someone on the Isles board also mentioned that Pulock has a no-trade contract so Lou cannot just move him to any team at will. He has been playing well this year being healthier and has a ceiling not yet reached. As he is signed to a team-friendly contract I do not see him being moved unless it is for a proven healthy scoring top 6 winger in his prime. The only move I can ever see involving Pulock is a trade to either Winnipeg (birthplace) or Toronto for one of their excess healthy scorings LH forwards with terms. A decent 2nd pairing RHD under 30 has to come back though if traded this or next season, with Mayfield possibly leaving in the summer and no one in the Isles system yet proven ready to take on a 2nd RHD role next season. Isles need to avoid any more deals similar to the Toews one.
Bo Horvat has a lot more value than JGB, even as a UFA, so the pick(s)/prospect added to the deal would need to be quite good. In that hypothetical scenario, Islanders wouldn't necessarily need to trade any "bluechip prospect" per se, but they'd need to part with a really good piece (plural possibly) to make up the difference.
Also, if Horvat was let go by Canucks, which RHD do you think they would be able to sign with the money freed up that way? Klingberg? I like him, but he's not what the Canucks need, at all.
As for the other UFA defencemen, while some could sincerely help the Canucks next year irrespective of handedness (Mayfield, Soucy, Gavrikov, among others), I really fail to see how they could match the impact Horvat has on the ice for the Canucks.
Even the whole of JGB+prospect+UFA signing for roughly Horvat's cap hit would potentially not be an advantageous deal for Vancouver provided that they intend to compete soon-ish.
So yeah, letting Horvat walk is a bad idea for the Canucks, even if you consider that they have two centers in Pettersson and possibly Miller who can show well in the top 6.
If they're smart they'll either manage to land some very interesting future pieces for him (which won't include Pageau), or then they'll trade someone else and just re-sign him given that he's a really important player for the Canucks.
And I say that as a Montreal Canadiens fan with no dog in the fight