Realistically, I think since Whitney wasn't the lead actress it would be ok to recast her part. It would damage the film to not include her character because it was about the sisterhood bond.

The big "what if" is how the soundtrack will play out. The previous one included Brandy, Mary, Toni, Chaka, TLC, Aretha and Co. and swept numerous awards but Whitney's Shoop made Billboard history and the soundtrack gave her two #1 hits. Those shoes cannot be filled and anything less in terms of the ensemble will be viewed as an utter failure.