Not challenges as such, but be aware that any fields you add will not be added to existing multimedia components until they have been edited. Also, if you have the schema set as the default for a publication and you add mandatory fields with no default value, then you will not be able to upload multimedia components via WebDAV unless you change the linked schema for the folder you are uploading to.
Remember that the Default Multimedia Schema will be BluePrinted right from the very top of your BluePrint chain (i.e. the Primary Parent). You may wish to consider creating a new copy of that schema in your schema publication, adding your metadata there, changing the default for your publications and updating your other schemas/code so that they can use both the old and new types.