I want something like a Firefox for launchers, with the same kind of addon support - replace any UX element, any animation, any function, add new ones, disable what you don't want, inject hooks where you like them, etc...

I'd love to see a notification tray that's customizable with widgets, search bar & voice control, custom toggles, context dependent adjustments (per-app or per-location quick settings and widgets, etc), IR remote control quick access, intelligent sharing menu (share screenshot / current open file / location / whatever), etc...

So far nobody has really gone for the whole scriptable sandbox approach. Mostly just layout customization over anything else.