Unfortunately, they already have a $10M+ winger and cap-wise that's all any team can really afford cap-wise if they plan on icing a championship team.
I wouldn't re-sign Marner or chase Rantanen. Tavares will take a big pay cut, freeing up ballpark $4-$6M on top of Marner's near $11M ($15M+ total). Knies' extension will eat up, say, $6M (just throwing numbers out here), which leaves somewhere around $8M-$10M (completely ignoring the cap increase for now) to get a 3rd line C, round out the bottom 6 with some veteran talent and have some cap space to play with at next year's TDL.
Losing Marner will hurt but the depth the Leafs can add will help immensely (PK specialist on the shopping list, for one). And they'll have enough cap to find a decent middle six RW for Tavares.
Of course this is all fantasy world talk as it's clear MLSE's biggest priority is not "just win" as Pelley tried to sell us, it's re-signing Mitch and secondly Tavares. That is it.