You’re not the only one who thinks this would be a great idea.
https://community.monzo.com/t/the-pulse-zero-funds-indication/5854?u=beningreenjam
At the moment though, I don’t see how this would be possible at least until direct debits, standing orders and regular salary income are introduced when becoming a fully featured bank. Without that information how would Monzo know when a balance is predicted to hit zero?