This feature would strongly decrease the risk of data-loss: An option to ask of more then X emails are about to be deleted (with a default of 10 emails). So deleting emails to the trash would normally not trigger a confirmation, but deleting a large number of emails would.
Background: In my organization I have multiple times observed someone losing their inbox (or another directory) that was abused as todo list, and a lot of important information was lost along. Finding the relevant emails in the trash along thousands of other emails was no actual option. They use IMAP and access the account from multiple devices. As IMAP is very poor on keeping track of the changes, no one exactly knows what happend ... howevery, most likely they pressed something like Ctrl+A and Del/Backspace on one of their devices, thinking they were typing in another window.