So our current format works quite well for automated tool usage. If you don't already know, it's basically a serialized HashMap containing class names and the attributes from the @Plugin annotation associated with each class. Normally, this file is generated through the Java compiler via an annotation processor. However, languages like Scala, Clojure, Groovy, and whatever others that exist that don't create intermediate Java code don't support annotation processing. Thus, the resurrection of the runtime package scan functionality.
What I'd like to propose would be some sort of human-editable plugin file format for those who may prefer to create their own. It would be nice for being able to customize your Log4j distribution by enabling and disabling plugins selectively.
As a Java guy, I'd propose an XML format because that's what everyone else does. Plus, there's the XML APIs already, so no custom file format processors or anything are necessary.
Note that the serialized file would still be preferred as the way to go since it's already there and supported. However, offering an alternative would be neat. Plus, the annotation processor could be extended to generate the XML (or whatever) file.