Not sure if you can get one pass that would include Slovenia, Serbia and Croatia with western Europe...double check that, though. Austria is a good gateway to eastern Europe, so if you could add that, it would probably help you out. Rail travel east of Austria is pretty cheap, so buying point to point tickets shouldn't set you back too far. From Serbia you could head up to Hungary and then west (I'm sorry I don't have my rail map handy, so I don't know what the trains are like, but check www.diebahn.de
for schedules throughout Europe. They don't have pricing info outside of Germany.)
From Austria to Paris is easy, though time consuming, you might want to consider a cheap flight from Vienna, and spend a little longer in the east before heading back. It's magical.