Problem:

Return command can only return predefined constants

return 12 

That means that you can not return result of other function (or just ony command)

// do not work
return function namespace:path/id
return scoreboard players get Player board

This feature will be very userful when creating datapacks.

Possible solution:

Allow return command to return result of any command. Like execute command handles and passes result of child command.

// possible syntax variants
return result function namespace:path/id
return run scoreboard players get Player board