I actually love what Archive of Our Own has made for the add-tags box (and characters, and pairings, and fandoms). There's not a drop-down menu, but when you start writing, the list of all tags somewhat alpha-related appears, and changes as you keep writing. For example, if in pairings you want to add Wilson/House, this happens:

+ when you have the first three letters (or more, or less -- this is just an example), all pairings with any form of wil will appear;
+ let say you already typed Wilson. the list contains all pairings with a character named wilson, including Greg House/James Wilson (yes, with the bolding)

You can add new entries to the list, but if what you're looking for is already there, then the chance of miss-writing something is much lower.