1) A built in trem locking system, self contained within the rear cover.
A simple vibrato lock would rule! First, the extra sustain would be nice. Second, not having the 5 other individual strings change pitch ever so minutely when adjusting the tuning of 1 string.
Third, I like changing tunings but not too drastically. Most of the time I switch from normal E tuning to Eb or Dropped-D turning. A spring loaded design doesn't lend itself to that because of the other strings going out of whack when tuning one string. Would be nice to just lock it, then I can change my tuning. Obviously, for more radical detuning, most people have to have their guitar specially setup for that so just simply locking wouldn't work.