Narender Singh form ForcePanda wrote a two blog series about using map collections in flows. The second blog makes good use of the new generic sObject support in flows coming in the Spring ’20 release
Part 1: https://forcepanda.wordpress.com/2019/12/17/how-to-create-a-map-collection-in-flows-part-1/
Part 2: https://forcepanda.wordpress.com/2019/12/18/how-to-create-a-map-collection-in-flows-part-2flow-map-methods-spring20delight/