The two people likely to succeed her are Kate Forbes and Humza Yousaf, with an outside chance of Angus Robertson (who I imagine will actually go for deputy).
Kate Forbes is an islander, and quite Christian, so would be good for the middle ground centrist voter, but I don't think the SNP membership are that keen on her as she has some quite Christian views at times that she broadly keeps to herself, but could still mean a tack to the right for the SNP. Humza is the one I think the membership would go for.
Derek Mackay was being groomed as Sturgeons successor until he, well, was caught doing some grooming of his own.