Hi Pierre
I just want to comment that XnView has been down a long road and many versions to acquire the favorable features and design we currently enjoy. XnView represents a consensus of many inputs and because of that, most users (I think) like the way it looks, works, and feels.
In light of that...
...please consider very carefully whether it would be advisable for you to "re-invent" the interface at this late date, even if the XnView MP project offers a tempting opportunity to do so!
IMHO,
all versions of XnView should look and behave the same way (within the restrictions imposed by platform specifics, of course).
EDIT
After making such a strong statement

, let me add that I personally really like the vertical tabs on the XnView MP folder tree sidebar to select between folders and categories. Hope they are implemented in the current version sometime soon!
Thanks for listening!