Before you posted I was going to ask if an external version control program wouldn’t be better then trying to do this through Blender especially for larger projects. I have never used one myself but it is something I kept seeing been mentioned IIRC the was a whole post on it on the tube animated short’s blog
Ah… thanks for clarifying. That said, it’s become apparent that this is about autosaves rather than versioning. I personally wouldn’t want to use an external version control system on an automatic basis (commit logs are too valuable to ignore, for me at least).
(Is anyone know a way to convert path automatically… :))
Solution found, in fact python includes a native function for that, os.path.normpath() 
Apart from that, I have a working version with handlers (I just hve to correct 1 little bug before releasing it), but the issue with modal operator still remains… Any idea ?
Again, a handler issue blocked me: it seems like when doing stuff inside a post_save handler, the filename is not updated… How can I update it, or at least get the new file name ?
(triple post, sorry)
In fact, I just made a stupid mistake: the post save handler was a pre save handler in fact ^^
So now there is the working version, tested on linux and windows: https://github.com/lapineige/Blender_add-ons/blob/master/AutoSaveIncremental/AutoSaveIncremental_v1-5.py
Ok, the Addon is working fine !
A video to explain why I wanted it. (in french)
Thx lapineige ! ^^
Just need to find if it is possible to auto activate it.
I forgot to post the last version, with some small code tweaks: https://github.com/lapineige/Blender_add-ons/blob/master/AutoSaveIncremental/AutoSaveIncremental_v1-6.py
In fact, you do’nt need to reactivate it when you manualy save your file (the handlers are doing that for you).
Except in 1 case: I stupidly forgot that it can be use with a non relative path, so it cannot deal with it for now, except if you relaunch it (and not all the time).
I’m working hard to fix it, but I have too few time for the moment…
Happy blending everyone 