At this moment can be found in this document from 2004:
http://www.stadsbyggnad.goteborg.se/prod/stadsbyggnad/dalis2.nsf/vyFilArkiv/LiesebUtvidgProgrSamr.pdf/$file/LiesebUtvidgProgrSamr.pdf
On the first page you will found out where the expansion area is, the parking area.
The plan is to build a building in the left part, to both reduce noise from the new park part. The building would contains restaurants, cinemas, shops etc.
In a interview from 2006, it was said "There is a possibility that a roller coaster in the dark can be added at the new area".
What Liseberg now is waiting for is to get the final decision where the new underground railway will be built. The decision will be official later during 2007. One of the choices goes just under the new area.
I´ll post more about the future expansion when i know more.