Defensemen are fairly easy to acquire via trade or free agency. All of NJ's current top four were signed or traded for, 3/4 of Colorado's top four were trades, same goes for 3/4 of Tampa's Cup winning top four.
The problem is the Sharks don't have the cap space or the surplus assets to even get involved in those kinds of negotiations. That's why they need to dump Karlsson regardless of the return as long as it frees up long term cap space. Also why Meier needs to be traded instead of signed to a $75 million contract. If someone is willing to take Couture at 5M/year he should be gone too. Then over the next two deadlines players like Nieto, Reimer, Labanc, Kunin and Barabanov need to be dealt for mid round picks if possible.
If they accomplish all of that then I think adding a starting goalie, top four defensemen and complementary forwards shouldn't be too difficult. They will have surplus picks and prospects to deal from and the cap space to accommodate players they trade for or sign in free agency.
The much bigger issue will be finding a 1C and 1D which can only be done at the top of the draft. If they don't get lucky with Bedard it will take many years of finishing at the bottom of the standings to fluke into foundational pieces like that. But dumping Burns, Meier, Karlsson and Couture would certainly help give them multiple kicks at the draft lottery can.