Let me give some background on the IXmlElement and related API.
In the early days of IzPack there was no XML parser in Java (1.2), so we went for a tiny library called NanoXML. This is where the IXmlElement interface comes from. Because it was a simple and flexible data containers, we opted to directly use the XML DOM trees to store data. Was it good or bad? At least it was very convenient.
Later on we refactored the code base to get rid of the unmaintained NanoXML, and take advantage of the JDK built-in parser. The adherence of the NanoXML to the rest of the codebase was high though, hence IXmlElement and related class serve as an adapter.
I agree that it would be great to further refactor the thing, but I am a bit skeptical about the proposed choices. XStream is not an option as it adds further dependencies. JAXB could do the job, but this is an API that can bite you hard at times.
If you really want to go this way then I suggest that you try to develop a proposal in a dedicated Git branch, preferably pushed to your GitHub clone. We can then easily discuss with the community on pull requests.
What do you think?