Probably late for this to be in the Z3 but here's what I hope for in a possible Z4 if it undergoes a major update

Better repairability
+ Easier disassembly - Screw-less and modular design: like in ASUS Chakram, the left and right buttons are separate from the main body. Screw-less alone is amazing. Makes cleaning more convenient.
+ Sale of individual parts - In case only some part breaks, like the PCBs, get a new one instead of replacing the whole mouse.
+ Hotswappable switches - Make it easy to just take out and replace without solder.

Wireless
+ Accessible battery (maybe an attachable puck at the bottom?) so it can be easily removed for wired. Easy weight reduction.
+ Qi Charging
+ Replaceable battery (the puck, or something more standard) since they deteriorate with time - Ties in with the "Sale of individual parts" point.
+ 8000 Hz wireless polling

+ Wireless dongle but works with bluetooth as well

Add 2 right edge buttons like the Roccat Tyon. More buttons where they can be realistically added without causing misclicks.

Optical or Hall Effect switches - The solution for double clicking.

Better mouse sensor with 8000 Hz polling rate

Lower priority wishes:

Weight reduction. Shave off some grams where possible. It's a heavy mouse.

Minimal DPI deviation

Paracord cable (unnecessary if wireless)

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I love the Z as is, considering I had the Z1 until it broke and kickstarted for the Z2 with a friend I convinced, but I believe these features would make it a much better mouse. 

I know the Z3 already has wireless. I don't actually want a wireless mouse (I much prefer wired) but more options is better, and said friend I mentioned is looking forward to the Z3. My wishlist is what I hope to be added to make this great mouse closer to perfect.