For me I would like the following
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More power with the same battery life
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Same 800p OLED Screen, but supports 120hz VRR
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Vita D Pad and switch placement with left trackpad (They can sell 2 models for classic set up)
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Official Dual Boot support with Windows specific Software for the same TDP Control etc and more driver support
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2 USB C ports
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More Color options (White, Gray, Black, Orange, Translucent)
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NVME 2280 support
My list:
I think everything else is fine. More case options is probably beyond the scope of what they really should want to do. A larger, faster NVME SSD is pretty much a guarantee anyways, no need to ask for that.
Micro not Mini LED is superior to OLED, more expensive though. Would probably double the price of the handheld, but there are plenty of people (like me) that would buy it anyways. Should only be an option, not the default, unless they somehow manage to do it at the same pricepoint as OLED; otherwise their sales will drop considerably.