Currently cache files are saved in $Home/kdenlive/ by defaut.
The idea is now to create one folder by project in $home/cache/kdenlive to save all project tmp files: thumbs, timeline previews, proxy files.
this will comply with XDG standards. Of course user should be able to specify a custom location
This is perfect but I suggest to let users choose the folder they want to use mostly for previews files.
When you work on a very big project you can create several Gigas of preview files. So it's always better to target them in a specific disk or, anyway, not on the same disk where you have your OS.
Then if you want to bring your project, with assets and rendering files, to another machine, you can just tell to Kdenlive where to look for them.
The only thing now missing may be for some users to change folder locations from the XDG standards to their own settings?
Remember some users need to select&remove a project and all its related stuff with it.