There is no such option now! Btw, since the topic says, "Dropdown values from DB lookup", are you storing your dropdown values in a DB? If you want to lookup from DB and populate the dropdown via a REST call see if this little DB to REST plugin via datamodel helps.
is there future enhancement planned around this from Alfresco to enhance APS? I am using lot of java delegates and task listeners to persist the data between Activiti tasks. so it may make sense to do DB query in java itself and populate json of the dropdown directly in the pre task listener class, just like I can do it for dynamic table. ... sounds like a relatively easy addition to the product?
I cannot speak for the roadmap. However data model feature in APS is a key part of the roadmap that can be used to persist and read data to and from external systems. So I would expect more data model integrations in form components such as drop downs in the future. As a customer/partner using APS, feel free to raise a feature request via your portal.