Most of GUI development is pure data handling (render state and react to interactions) where a strong functional toolset is much appreciated (lambda, pattern matching) and some parts are well done with closures (where OO comes in)
Ultimately I would love to have a GUI development language that allows me to use one set of language features and constraints when I really just want to use functions on data immutable data, strong pattern matching and ADT for modelling) and another when I need strong closures, with message based dispatches and reactivity.
Basically, whatever React does, minus the constraints of requiring JS compatibility.