The following are links to external resources that have either inspired, or will assist in, the design of LevML.
The Web3D Consortium - Developing X3D; another standard for 3D using XML.
GameDev.net - Creating Virtual Worlds: In this article, the author creates and demonstrates a Perl-based API for level design. A series of nested cubes is used, thus providing what appears to be a simple framework to both implement and conceptualize. Conceptualizing how one's results would render might prove to be rather difficult in a text-based form, but this cubic structure might be a workable solution.
The CrystalSpace map format: CrystalSpace is a popular open source 3-D engine and game SDK. Its maps are XML-based and, while LevML is intended for a higher-level (no pun intended) in the design process, certain CS elements might be borrowed as inspiration for lower-level primitives. It is also an additional map format worth targeting.
Quake MAP Specs: This will probably be one of the first supported output formats.