One of the TV critics I read had a pretty great idea - bring the show back without Roseanne's character, but never have the rest of the family address it. As if Roseanne just stepped out to go to the store.
...another one suggested they kill her character offscreen like NBC did to Valerie Harper's sitcom "Valerie" in the 80s. When she wanted too much $$, they canned her, killed her character and changed the name of the show to "Valerie's Family" (later "The Hogan Family") and squeezed a couple of extra seasons out of it.
The same guy suggested that they could represent Roseanne by putting an urn on the mantle with her ashes in it, which the characters could talk to as if she was still there