for an developper with J2EE experience, it is the best to learn ABAP because then he can improve the interfaces for communication between ABAP backbone and the UI in the Portal.
All other options are going more into the Application itself,
but this is an hugh, possibly new area, (with 3 parts:
OS like Windows, Linux and /or UNIX
Database like ORACLE UDB and / or MaxDB
and SAP-Basis itself )
were you have to install SAP with ABAP and J2EE, Monitor Operations, Printer Setup,
changes and upgrades with OS DB and SAP, Support the Users, fix errors, an