The dark horse team to watch out for, for a potential Marner offersheet are the Philly Flyers. IMO
In 2006 they attempted it on Ryan Kessler, and in 2012 they attempted it on Shea Weber, in both cases their original teams matched and kept their own players, but it shows that Flyers organization is not afraid to use this CBA tool, and got past RFA players to actually put pen to paper also and known for their aggression is this area.
Philly also has > $33 mil in free cap space at present amongst the most of all NHL teams (only Colorado, NJ & Ottawa have more) and do already have their key vets already under contract for 3+ years.
Flyers likely to go aggressive after the first wave of UFA players (Panarin/Duchene) and then depending on that outcome might be primed for an OS attempt on Marner.
They might also be willing to package together in trade assets to try and acquire Marner if the Leafs are struggling to sign him as they have the luxuary of some younger Dmen assets and forwards that might be of Leafs interest also. (ie 11th overall pick this year + Travis Sanheim and Travis Konecny/Nolan Patrick etc) all former 1st round picks.