It would be super weird to completely recast the character, but it might be horrible to kill the character off so soon after Jack Thornton died, although it should be said that When Calls The Heart is not immune to tragedy, having kicked off in Coal Valley right after a mining accident killed many of the men in town.
The other option would just be to have Abigail mysteriously leave, but again, even that could be clunky since we are midseason right now.