A preamplifier is more than a volume control. It is the command and control (C2) center for your audio system, so it is important that it have all the features you are ever likely to need. The core line amplifier is straightforward, it's implementing all the "uncore" stuff properly that's the real problem. Each feature comes at a cost of space and complexity. Put everything in there and the job is truly massive.
Brainstorming session in progress.
Voltage Gain [include - 12 dB?]
Output buffer [include]
Volume Control [include - goldpoint V24 50k]
Input Select [include - 3, (phono stage, tuner, cast]
Mute Switch [include? (output switch)]
Gain Switch [no]
Balance Control [useful, but no - maybe +/-2 dB fine trim?]
Mono/Stereo Switch [interesting, useful for mono LPs, but no ... ok maybe]
Tone Controls [no - unfairly maligned, but too much trouble]
Recording / Buffered output [no]