You could use a Wait For Entry Change activity and configure it to wait until the needed metadata has been supplied.  After the Wait For Entry Change activity, add a Rename Entry activity to perform the actual renaming.

Note: The Rename Entry activity can rename any entry used in a workflow, not just the Starting Entry.  So, for example, it could rename a folder to which an entry was routed with a Route Entry to User activity.

You could also use a starting rule to achieve a similar effect (using an Entry Changed starting event), though you'd need to make the rule specific enough to only run on certain folders.  Depending on your repository's organization, configuring this starting rule can be anywhere from trivial to very difficult.