Converting ArcGIS (*.mxd) file to QGIS (*.qgs) file?

This looks like the sort of thing you want: https://underdark.wordpress.com/2011/05/27/converting-mxd-to-qgis-project-file/

On Wednesday, Allan Maungu announced MXD2QGS, a converter that exports layers from an Arcmap 10 document into a Quantum GIS project file. The tool is built as an ArcToolbox and can be downloaded from the blog.

The only working download link seems to be https://sites.google.com/site/lumtegis/files/Mxd2Qgs.zip

As of August 2017 this tool also appears to be available at https://github.com/fitnr/mxd2qgs

There's also one for doing ArcView 3.x files apparently: http://gix.sourceforge.net/

GeoCAT offers a commercial ArcMap plugin called 'Bridge' that can convert all of your ArcMap symbology to SLD (there is an open source alternative arcmap2sld). In QGIS you can import SLD and use this as style for QGIS layer. In bridge the tree structure of the TOC is exported as WMC, and the data as shapefiles (including metadata .shp.xml) and tiff's.

I would want to create a tool in QGIS that can read the WMC, sld's and data and create a QGISs project from that.

The SLYR plugin will convert MXD to QGIS. Unfortunately, as of 2020, this plugin is not open source and is quite expensive. A free community version exists, but has rather limited functionality.