Form factor: 66% and 75% is not 60%. FC660, Choc mini or Laptop pro never been options for 60% lover.
TKL users are sure they never or very rarely use tenkey; meanwhile,
60% users are sure they never or very rarely use tenkey, F-keys and navigation cluster.
They are happy without those keys, if their keyboard has unused excess keys it is totally dealbreaker.
Fancy things: USB ports, back light and such are evil.
Totally marketing sake! I don't wanna pay for them. Not absolutely dealbreak but still meh.
... IMO
Layout: Never one fits all. No one can design universal layout for everyone. Programmability is required.
1) Reprogrammable controller.
The keyboard will get to be fully programmable by user community endeavor if MCU datasheet and toolchain are publicly available. No need extra customer support cost for manufacturer!
Widely pervasive Atmel AVR or ARM Cortex is preferable? I think we'll happily pay the cost for that if it is like $5-10.
2) Or manufacturer offers configuration tool.
Good tool will be a great marketing hype. Manufacturer should give expense real fucntion like this not fancy things.
3) Or we can remap all keys with tool like AHK if Fn key itself has keycode.
60% keyboard should have a (DIP SW) configuration which gives a keycode Fn key and offers plain layout without Fn layer. Users will be able to fully configure their own layout including Fn layer themselves with tools. This doesn't seems to cost manufacturer that much.