La Défense is truly unique with its crazy maze of passages. This is a long-term challenge that will keep you and probably many other mappers busy for a long time...

I would say start with what is most important - the RER and metro stations, the motorways, the main pedestrian routes on the surface and any landmark buildings such as the Grande Arche or CNIT. After that, the more passages you add, the better.

Unfortunately, neither JOSM nor Potlach is designed to handle such vertical complexity. It will be very hard for you as the cartographer and for any map user to keep track of what crosses what at what level... but that is how La Défense was designed - not our fault.

Thanks for tackling this area. The next time I am in Paris, I sure will be glad to have good maps of La Défense available.