Today, I am sad to report that despite my reflexes not being what they used to, I now wait upon file explorer to do its thing on a regular basis. I could play folder tetris at lightspeed with that thing, because it actually felt faster than I could ever go. Sure, everything loaded slow off disk, but once you got your basics into RAM it felt so much faster by comparison to how Win10 responds today. Who remembers how snappy the experience was on a fresh XP install? though that can also have false positives if the changes are by the OS itself). A properly integrated Windows 10 solution would be to map existing files to VM so that it reuses 99% of the stuff that would be the same and, if needed, do a copy-on-write in cases where these files are altered inside the VM (it could also help to see what exactly the programs inside the VM touch, sort of a system-wide diff. It is kinda/sorta possible to do that now using something like VirtualBox, but you need to create a full Windows 10 installation (which takes several GBs), it takes time to install (you can save some time by exporting a fresh installation to an appliance - at expense of more space - though you'd also need to disable networking otherwise after a while the moment it notices internet will start upgrading) and it can't use the computer resources properly (it runs much slower, you have no access to the GPU, etc). This would make it easy to experiment with applications you do not fully trust or simply trying out applications in an environment that wont affect your real working one.

I'm not sure if this is what you have in mind (because sandbox/container can mean several things, including things i wouldn't like to see like limiting what applications can do to each other) but one thing that i'd like to see is something like a 'desktop chroot': being able to right click an EXE (or zip/installer/msi/whatever) and select 'Run this in fresh Windows 10 VM' where the application runs in a VM (either inside its own window or seamlessly, could be an option) as if it was the only application installed in a fresh Windows 10 installation, optionally with a way to make this VM permanent as a desktop shortcut, until you decide to delete it and remove the entire thing as if it never existed.

Windows 10 weather tile not working