First off, I think this software is fantastic as is.
Second, I would like to see support for other databases. For example, SQL Server and MySQL. It would be nice to be able to configure MPI to connect to a database of your choice (not necessarily the Access db currently used as the backend). Maybe even connect to an ODBC or OLE source? Or Amazon SimpleDB or Amazon RDS? Of course, I realize this might require some substantial development, but my initial impression is that the database structure itself is not overly complex and doesn't rely on proprietary functions.
Ultimately, I would also like to see a more robust database that could support multiple users/machines and perhaps handle permissions. I have a moderately big home network in mind where photos are saved primarily on a network drive but are accessed from multiple desktop/laptop computers. It would be nice to have a shared database. I'm sure that the software could be extended beyond that audience, however, if successful.
Just some ideas....