a) Detachable RCA cable is a must. Same for the ground cable, just leave the RCA connectors / a screw there and let people use whatever cables of whatever length they want. Using built-in cables may be cheaper but it's not worth it, you don't want users to have to manually replace damaged cables with a soldering iron.
b) Built-in phono stages are practical but in most cases they are cheap and hurt sound quality, also many people would likely want to use their own, if you decide to put one, make sure it's a decent one and allow for it to be completely bypassed. A nice plus would be to also allow switching between MM and MC, since the pre-amp circuitry is a bit different depending on the cartridge + maybe add a headphone jack as well.
c) 78RPM support would be a nice plus for some people.
d) You'll probably go for a switching supply, make sure it's a decent one with proper noise filtering, especially in case you add a built-in phono stage.
e) Rubber feet suck, go for something better instead (e.g. noise damping cones/spikes w pad), it makes a difference if you don't have a proper stand and the upgrade is a relatively cheap one. They also look much cooler :-)
f) Throw in there a replacement belt just in case.
In general I'd always go for build quality instead of added features, if you can't get the raw sound from the LP without compromises any other stuff such as built-in amp stages, ADCs with usb ports, auto-stop / auto-park, 45/78RPM support don't make much sense. I have an old Thorens turntable, its build quality is phenomenal, from its suspension system to the motor to the spindle. It doesn't need any extras, it's only weak point were the built-in RCA cables that used to be common back then. There are so many turntables out there with "cool features" that can't even play decent stereo (not to mention some of them even damage LPs), on the other hand there are so many over-priced turntables for no reason, targeting "audiophiles" who don't know what they are paying for as long as it's featured on some blog/magazine. Start from a solid turntable and then add the extras, to me a turntable with a good motor, tonearm, plinth and power supply, a decent built-in phono stage, that can properly support MC cartridges, below 400$ would be a good product. Especialy if I could buy it in EU at this price.