Add a 1 button apply all Filters to existing emails somewhere on the main page.
When creating filters for the first time, you have to either go to the settings menu to create and run them or you have to click the check box of an email > click the move the folder icon > select the folder > select 'Always Move sender's emails'. To apply that filter to past messages, you still have to go into filter settings, and run that 1 filter against the entire inbox.
If someone has a mailbox that they've never organized, this adds a lot of unnecessary back and forth between the Inbox to select an email and create the filter using the Move to icon and the filter settings to run that newly created filter against the entire inbox.
Adding a 1 button solution that applies all existing filters, without having to go into the settings menu, would make this process a lot easier.
Filters would need to run much faster, so that users don't just click the button 10,000 times, when they don't see it working immediately.
Or you could add a progress bar similar to Outlook's rules, to let the user know things are happening and to wait patiently.