Website design – Alternatives to the drop-down list when the possible results are unknown

For quite some time I have been searching the web for alternatives to a drop-down list in the data assignment (that is, during the capture of data that will be saved, NOT when looking for some data).

An alternative that is commonly suggested are the "autocomplete text boxes", but only when at least part of the data is known.

Imagine, however, the selection of a city in a specific country. It is possible that the user does not know what they are looking for specifically, therefore, it lists all the cities identified by zip code or their name.
For that scenario, a pop-up window with full search capabilities can be used, however, the process slows down again.

What else does your mind do? What are you commonly using? Do you know any important side that has found a great solution for this?


[Update]

After finding that I could not correctly describe my question (sorry for that), I try to clarify the use of a screenshot for demonstration purposes.

Screenshot

Assume a form to add new clients, where the name of the client and its main contact employee for that new client is captured.
The employee (assuming a large company with thousands of employees) is identified by four attributes:

  • gender
  • First name
  • surname
  • branch

In the screenshot above, I showed them in a drop-down list, but I think it's obvious that this list is no longer usable. Since the employees differ in 4 attributes, I believe that a self-suggestion is not really usable. So, what else do you commonly use for that purpose?

I hope the question is clear now.