The DMN variable support (inputs to decisions) probably shares much work required for the Declarative Process Variables feature, in that DMN input variables would likely need to be all of the list of expression types in the Declarative Process Variables, i.e.:
static values
Flows for APEX Process Variables
APEX Page Items
SQL statement returning a value
?? SQL Statement returning a colon delimited list of values
The DMN variable support (inputs to decisions) probably shares much work required for the Declarative Process Variables feature, in that DMN input variables would likely need to be all of the list of expression types in the Declarative Process Variables, i.e.: