I saw this in a thread from 2015 but wanted to re-start it. Would love the option of being able to select multiple items from a drop-down list. This is handy for improving the database functionality. Many times I have sheets with dozens of projects/rows, and I need to list multiple values for a single attribute, then query based on a selection of those values. I can try to remember the values that I am using and enter them manually, but if I get the wording on one wrong it of course will not query correctly.
Overall for SS to really shine need to enhance the database function.