YES PLEASE!!...
When I feel like adding a new mastery page in between existing pages I eventually do end up planning on a total overhaul of all my mastery pages. This can take a deceptive amount of time and has happened many times before...
When they did a performance upgrade on rune/mastery pages they didn't bother to do this because they weren't sure how to implement it. Apparently they were trying to use some wonky arrow button to swap pages. Why can't we just drag and drop..? If I were them the interface would list the pages top to bottom, so it's sort of how you'd see it in champ select. This way there's also potentially space to display the mastery page name and do away with the numbered button. The actual runes/masteries would go to the right...