Wikimedia Bugzilla is closed!

In your (Yaron's) words:

"A new input type for an enumeration with multiple values allowed - 
instead of a listbox or checkboxes, have each additional value be 
selectable using a separate dropdown, with the ability to add or 
remove dropdowns using buttons."

Synopsis:

On a form, a declaration that might be similar to

<pre>
'''Contributors'''.
{{field|Names|input type=combobox|multiple|add_button_text="Add another name"}}
</pre>

would result in a form showing

<pre>Contributors
[Add another name]  <= button 
</pre>

Action: hit the [Add another name] button and select Rick Jones

<pre>Contributors
Rick Jones        [Remove] 
[Add another name]
</pre>

Action: hit the [Add another name] button and select Bruce Banner

<pre>Contributors
Rick Jones        [Remove] 
Bruce Banner      [Remove] 
[Add another name]
</pre>

Today, this can be done with SIO, a separate template, and the multiple keyword when used on a Form:page.  However, that reduces the flexibility of the form layout since the separate template needs to be outside of the "main" template on the form.  There are a few other advantages / disadvantages that I can bring up in comments if necessary, so I can keep this as short as possible.