Yes! It would be absolutely amazing if Agenda would include ‘drop-down options’ (or something similar), sort of like when entering args into a class within an IDE. This would work great with x-callback-urls, but may even work better with custom tags.
So, let’s say you use a “priority” or “importance” tag, we could automatically get the values presented as soon as we type the “(“ and Agenda could even close the task by adding the final “)” after the selected option. An example of this would be to say you tap “low” from the list, Agenda would already have “#priority(“ as that would trigger the options, so Agenda would add the selected option and close the tag by adding “low) ” (note the intentional space as the last character here).
I do realize that some form of enumerated values would be necessary, but I also believe that even if we had to import an Excel spreadsheet or a CSV file containing those values, the majority of the users would be tech savvy enough to easily pick up its use. It could also be integrated into a menu tab (as a premium feature of course), for example by having two columns; a column for the custom tag on the left and a comma separated list of values on the right. Sequential numbers or letters could be separated with a - to simplify entering many numbers as in the case of a percentage (complete on a project). So, one could enter “#completed” in the left column and “0-100%” on the right. This is just a quick first thought, but being how powerful Agenda already is, it could really go anywhere! It has already replaced a rather large handful of daily driver apps for me!
Update: After I saved the post I saw that the backslash I placed before the dash disappeared in my enum example. I think it would be necessary and you may too, so please just pretend there is one. Thank you. 
