Aside from the answer I've given to you in the other thread (what was the point of starting a second thread about it?) the only limit is your imagination.
You could design and build a cart copier, printer interface, hard drive interface, CD-ROM interface - any kind of peripheral you want, really. All of the necessary bus signals are available on the connector, as I've already said it's just a "male" cartridge slot.
There's lots of technical and programming info on the SMS at the SMS Power site and on Charles MacDonald's page. I suggest you start there if you're serious.