Meanwhile, you have for example Adobe Analytics hier variable, which would benefit from being able to reference an array. As of right now you just provide 4 fields and i can reference 4 separate data elements (or hardcode 4 separate values). But in practice, I can have more than 4 levels in a hier variable.
I think it should be a pretty easy update to the Launch interface to just have the hier[n] variables be a single field to reference a data element that returns an array, and a field to specify a delimiter (already implemented), and then under the hood, check if the returned data element value is an array and then join by delimiter and call it a day.
This same concept can also then be applied to adding list[n] variables in the AA extension, and also for list prop syntax, and also for.. well, any field, really. Maybe put a little checkbox next to it to flag to expect an array and then make the delimiter field appear if you check it.