Because of the fact that you can never be sure what word processor/document reader someone on the receiving end of an email is using, it is a common practice to email a document in multiple formats (such as odt, pdf and Word XP doc) simultaneously, to almost guarantee the recipient can read your attachment.
I think the ability to choose multiple file formats (creating multiple files) when saving a document would be excellent. Of course, you could just save it 3 or 4 times, each time with a different format, but this is time consuming and could be easier. It can also be hard to remember which formats on your list you've already created a copy in.
My suggestion would be to offer checkboxes for each available file format, and when checking multiple formats, an option to create a subfolder matching the name of the file would be nice. There could also be an option to automatically tar or zip the output into an archive.
While it is always best to suggest that a friend use an excellent free software document reader that can read every format under the sun, in the case of a teacher or business associate it isn't normally appropriate to suggest that they install new software just to read your document.
A plugin that achieves this could be made for openoffice.org, abiword, koffice, etc.