For Erik Karlsson the only off limit are Hall and Nico.
No one else is untouchable. For a player like Erik Karlsson, a type of player thay never becomes available basically, top 3 d-man in the league, you gotta give up to get.
Ottawa would probably want Bratt, Walsh, Wood along with high draft picks. And I would still do it.
A realistic deal I could see Ottawa wanting and the Devils potentially willing to bite the bullet on would be:
Bratt or Wood + Zacha + Severson + 2019 1st for Karlsson WITH an extension.
Hall - Hischier - Palmieri
Johansson - Zajac - Wood/Bratt
Quenneville - Coleman - Noesen
Seney - Boyle - Anderson/McLeod
Greene - Karlsson
Mueller - Vatanen
Butcher - Lovejoy/Santini
Only issue with that is it guts our forward depth, especially at C. I think we would likely try to get one of their forwards back in return or add someone else. Pageau would be ideal, but not realistic, Dzingel wouldn't be a bad option, and Smith would be realistic, but not ideal.