Theoretically, you've got the following:
Rielly - ?
Dermott - Holl
Sandin - Liljegren
Granted, this is makes the assumption that Dermott, Sandin, & Liljegren are all ready to jump to the next level of their development, but I'm fairly confident in all of them. Sandin & Liljegren look phenomenal at the AHL, and both have plenty of experience there, and I honestly wouldn't be surprised if we see one, or both, of them in the blue & white in the 19-20 season for a stretch. In the same way, Dermott has excelled as the team's #5 defender for what is now two full seasons of hockey...he's shown he is more than capable of driving play at an elite level on the third pairing no matter who his partner is, but the question is, can he step into a special teams role and be a good 2nd pairing guy? I believe he can...he probably won't produce elite results, but I think he'll tread water and gradually get better. He has a lot of tools to be a good defensive player, but there are still a lot of rough edges.
So that leaves a gaping hole on our top pairing, because as good as Holl has been, and as much promise as Dermott/Liljegren/Sandin show, none of them are top pairing material. You probably also want to fill the gaps by adding a solid #3 guy, in the case that Holl/Dermott isn't second pairing material next season, but that move seems less pertinent.
I think Toronto has a few options.
A) Re-sign --> They could potentially bring back Barrie or Muzzin (or both, with other roster moves), but the question is, for how long, and how much? There's no question, both players would add lots of value to the roster, especially Muzzin. I would be perfectly comfortable with Muzzin/Holl as a shutdown pairing, and then Rielly/Dermott as an overwhelming offense pairing, and Sandin/Liljegren as a sheltered third pairing. I'd be less comfortable with Barrie coming back, because then I don't think Toronto has even one legitimate potential shutdown pairing, and is somehow even less defensively adept than last season. But if Muzzin especially (will be 31 in the off-season) wants a long deal, is it worth it?
B) Trades --> This is something that could even happen before the 2019-20 season is up...it happened last season when Toronto acquired Muzzin. Maybe Toronto will trade some of their excessive forward depth for defensive options. Some names I've seen floated that seem interesting are ones like Savard & Andersson...somebody who drives defensive results well, and is a right handed shot preferably.
C) UFA --> The other option is that Toronto uses the money from Barrie/Muzzin/Ceci walking to pursue a different big fish...a lot of people like the potential of Pietrangelo coming to Toronto, which would be pretty sweet, but that contract would likely be very, very hefty.
