It will probably need to be someone making 4-5m (or willing to sign at that) if the ask is multiple prime assets, otherwise it's not really worth it for them. The cap needs to go well above 4m for the Leafs to make a that kind of addition comfortably. If they aren't worried about the future implications, then I think the cheapest in terms of acquisition cost between Meier, Toews, ROR, or Horvat is who they could pursue. This team will probably need a defensive upgrade too. If Muzzin isn't out next season, then there really is no room (edit: it seems even more than likely that his career is done, but we'll see what they say at his next evaluation in February). So probably a relatively low cost rental that could be interested in resigning is the best option.

ROR is the one I would target, I think he'll be the best value. Maybe more expensive to acquire than Toews but has more left in the tank and less cap. Horvat sucks defensively and Meier has an absolute horrible QO next year making him impossible to keep.

I like Toews because if he's waiving for Toronto, then the cost could be lower but more importantly there is a chance at retaining him, so that's why I prefer him over ROR and Horvat. I also think that he would be more willing to play on the 3rd line if need be, vs ROR, Horvat, and Meier who would all also want prime powerplay time. Toews and ROR could both come on when Tavares is tired and replace him as the net front presence, I think Horvat could too but probably won't be enough minutes for him. Meier's fit in the top 6 is obvious, giving him PP time might be a challenge, acquiring him could be good in the sense that they could recuperate some of what they give up.

At the end though, if I was Dubas, I would be pursuing the cheapest one in terms of cost of acquisition.