Have recovery options for deleted lists and tasks
It would be nice if deleted items to go some sort of trash staging area so that you could recover them if you change your mind or deleted by accident, or if a shared used deleted something that you didn't want them to.
Flask needs overall better error prevention around deleting lists and tasks. Confirmation options ("Did you mean to delete this? It will be gone forever."); an error recovery ("Undo" flashes for a few seconds after deleting something in case it was an accident or you change your mind); permissions (different users can view, complete, delete, etc.); and a trash folder (where things are stored for a short time and recoverable before they are deleted forever) would all be great improvements to address this deficit.