Is it possible to develop a LLVM front end development kit which support easy language definition and creation with IDE integration like Eclipse Xtext, JetBrains MPS. All the needed C++ integration code should be easily created through the kit. Some other similar projects from which you can get ideas are DMS Software Reengineering Toolkit, Rascal MPL, TXL, ROSE, Coccinelle, etc. where some projects are source to source transformations. Source to source transformation ability is much welcome where other languages can easily be converted to Clang.
Ultimately there can be a definition of the grammar of Clang also to experiment or use as a basis of implementing new c++ language features and other language related projects.