Build a Visual Studio add-in to design workflows offline in a descent editor
The only good editors I have ever used have been desktop applications. Visual Studio, InfoPath.
These offer far more advanced user experiences to author solutions.
They are inherently built for offline and they are more stable and offer great keyboard support to build robust solutions.
The Nintex web based designer is terrible to work with and no developer should have to deal with the design experience you offer in 2016.