Floats landing gear if you wanna fly above water. That is my only non-genuine accessory I use and only if flying above water. Of course extra OEM batteries.
I'm not using filters, I prefer to deal that matters in post-production.
Sunshade for your screen.
Landing pad for dirty spot unless you quickly learn take-off & landing from your hand...
Carry-on data bank storage if you have many batteries and wish to do long shooting especially at high resolution