Hmm, so it seems that my thinking is going in a different direction.
Yes to the iPad Mini - ABSOLUTELY!
Its a must and next to it is a good (matt black inside) Screen Shade that you can just clip together and fit behind the iPad. A godsend in sunny weather (and not really needed in the UK as there is no sun ;-))
Next on my list was an Aluminium Arm that holds the main tablet clamp. The original one is a bit wobbly for me, so it was swiftly removed.
A Chest Strap is also a good investment as you do not need to hold all the weight of the Remote in your hands all the time.
Gimbal Clamp ??
Why .... as there is one already included in your set when bought new. And the original DJI one IS better then some of the aftermarket ones.
A light but tightly fitting UV filter is also a good idea.
DJI Rucksack .... I have both, that DJI one and a good Waterproof Case. Makes transporting all those bits easier.
Carbon Fibre Propellers ..... makes the Phantom just that little bit more stable.
And as I am a perfectionist, my next thing will be a Propeller Balancing Jig ..... as any pilot knows ... a badly balanced propeller will cause a lot of stress on the motors and in turn this will also transmit the vibrations onto the rest of the body of the aircraft .... just look at all those "hairline cracks in the body" videos on YouTube.
Other than that .....more batteries, more batteries and more batteries ;-)