If your wanting to spend money on worthwhile things for your bike (without costing a fortune)....id firstly ensure everything is serviced (fluids, plugs, filters, pads etc). Then make sure your on a decent set of tyres, upgrade to steel/braided brake lines. Get a qualified mechanic who knows suspension to tune it to your weight and required needs. Then if your wanting to spend more could go for exhaust (slip on is cheaper), full system if your wanting to chase some more hp and mix with ecu upgrades/adjustments as well as air filter etc, changes to gearing can give more low end performance too at little cost as well. How much you wanting to spend?
Stompgrips, stompgrips, stompgrips. (Some prefer Techspec tank skins, I've not tried them)
Swingarm bobbins and a 'paddock stand' for easier chain oiling etc.
Rear hugger to avoid as much muck getting flung under the back of the bike
Tail tidy + integrated tail light (aesthetics)
Exhaust can (bike dependent - aesthetics / weight)
Double bubble screen (if you are tall, means you can actually see the gauges)
I have all of these.
Suspension and tyres should be viewed more as consumables than mods IMO
I've wound up modding my last 7 bikes, including my '15 ZX636. My first couple attempts were mild. The last 4-5 projects have been much more substantial.
What's your motivation? Something to be a little different? A lot different? Just cosmetic? How long (time) do you ride? How far to you ride (time X velocity X distance). Do you commute & and what the traffic density?
My average day ride is over 250-miles; but my commute is <10-miles. During the summer I take two different long trip, each >2500-miles. What I've learned is the rider - You - must be comfortable. I always start with the usual suspects: dbl bubble windscreen; adjustable rearsets; fully adjustable clip-ons (Kawis have much more limited options with this one). I affix the usual frame/fork/swingarm sliders ( I totally love Woodcraft on this count). I also wire my bike up for Powerlet GPS, phone, & heated jacket. BMC air filter, Yosh, DJ-V.
Then I go for bigger pocket items like suspension, as finances allow. Bigger & better brake rotors (I only go with Brake Tech Axis 320mm)