Zoner Draw is a good and easy to use vector graphics editor. Its development stopped several years ago and the last version (5) was released free of charge, which makes it a good choice for students, beginners etc. Unfortunately it can save documents only to its own format (ZMF), so it would be great for LibreOffice to support importing from this format.
There are two variants of the format: one is used in the recent versions of Zoner Draw (4-5) and another, based on OLE2, was used by older versions (2-3). The most important parts of both formats have been already reverse engineered and available in OLEToy, but a bit more reverse engineering will be needed to reveal the missing parts.
Similarly as for other import filters implemented in LibreOffice (libcdr, libvisio, libmspub, …), a C++ library (libzmf) based on librevenge will be implemented.