Single & Multiple Select from Lookup or Rollup -> Arrayjoin(values)

It would be really useful if there were a way to dynamically create single and multiple select options using the rollup function on another column. For example, in one of my tables, I have a list of geological formations each of which has a set of dominant lithologies. A given formation might consist of 1-4 different lithologies. In another table, I have a list of rock samples. I link each rock sample to the geological formation from which it was collected, and it would be great then to be able to choose the exact lithology of the sample from the 1-4 dominant lithologies listed in that sample’s parent formation, rather than choosing the lithology from the list of all lithologies (there are hundreds).

I can add a rollup column and format it using arrayjoin(values), which gives me a printed array of the 1-4 values of the linked record on the other table, but there is no way then to take those array values and turn them into single or multiple select options for another column.

I imagine this would be exceedingly useful for people besides me, especially with use cases where a user first chooses or links a country and then chooses a state/province, or first a state/province and then a county.

TLDR:
Let us use array values from the rollup formula to populate single/multiple select columns.