Hey there!
My custom touch controls for the arc camera are getting better so I would like to share my progress with you. It’s kinda cool of controlling the scene this way. 
It’s a Google Earth touch control clone - the ugly one 
The solution is based on HammerJS. I am working on removing the depedency. The only thing it still uses from HammerJS is the event system. I am also planning to add some inertia to get a smoother feedback.
The debug gizmos can be turned on and off by calling setDebugMode(state) on the input class.
The input can be added to the scene like this:
// remove mouse pointer input
camera.inputs.removeByType('ArcRotateCameraPointersInput')
// add hammer js input
const hammerJsInput = new ArcRotateCameraHammerJsInput()
camera.inputs.add(hammerJsInput)
hammerJsInput.setDebugMode(true)
camera.attachControl(canvas, true)
I hope someone will find this useful!
R. 
DEMO:
REPO:
still beta though… Have a nice weekend everyone!