Many apps already have this. Your browser can restore windows and tabs on restart. Your $EDITORs save work in progress and let you restore on restart (well, VIM does). VMs can get paused and saved. These are by far the most important apps to have restored. Sure, I'd like my tmux instances and shells and stuff restored too, but I script all that setup anyways, so I'm good there. So, really, it's just a matter of integration.