The issue with Toronto is going to be cap space for 2020-21. Marner's contract is going to be (deservedly) rich and Kapanen and Johnsson are all up as RFAs as well. On the plus side they are all non-arbitration RFAs, so the team has some leverage there, but they are certainly not going to be in a position to add any extra salaries (and pretty assuredly going to have to let Jake Gardiner walk as a UFA). Until that Marleau contract is up at the end of next season, Toronto is likely going to have to look for cheap options from within for that RHD hole, unless another team as a trade partner is willing to take a bit of a cap dump in return (eg. Zaitsev).
Weirdly, looking at the Leafs cap situation the only bad contracts I can see are Zaitsev, Marleau (and it only has one more season) and Connor Brown (and at $2.1 million for one more season, even that isn't so awful). Just the nature of today's NHL that you can't really stay under the cap without leaving a couple holes on your roster... well unless your team is in a state with zero income tax and thus players will sign for less cap hit because they still take home the same amount (cough, Tampa), but with Ontario having a very high tax rate that just isn't an option for the Leafs.
However, good on the OP for coming up with a creative hockey trade where the values are mostly in line (at least enough that there are split opinions for fans of each team involved).