Depends on what you will use your camera for, but i would reccomend to make sure you have enough batteries or an other way of powering it (Those nikon Batteries last from about 45 min to 1Hour 15Min, so if you plan to shoot for extended time, Batteries will run out surprisingly quickly, unless you use it on an power adapter all the time for example for Studio use.), and also enough Storage. The SD-Card can handle quite a bit of .mp4 footage, but if you end up saving as DNG, it will fill up in no time, so a bunch of big spare SD-Cards, USB-Memory or an SSD (SSD will need a special cable to use it) to store all your footage.
Also depending on the Lenses you plan on using on there, Adapters might also be a thing to think about.
And something that will also improove overall experience, if used for Filmmaking purposes, is an Good External Monitor, which will allow you to judge exposure / Color better and can be viewed from any direction.
Trigger cables can be usefull, especially if your camera is in an location where it is impossible to trigger it without, for example general danger or Distance in Multi-Camera-Setups, but are not crucial for general use.
Apart from accessories for the camera itself, i would make sure to be able to light up whatever i am trying to shoot bright enough. The amount of Light needed for Filming Highspeed is easily underestimated! So one of the most important things to get when buying a highspeed Camera is actually lights.
