Custom Chromatic Tuning
Two different aspects:
1.) Eliminate the per-string pitch limitations in the current custom. There's no reason to prevent (say) the 6th string being D2
2.) Each string should be tunable to an user nominated frequency, perhaps allowing a combination of reference pitch and cents to define. This would support people interested in microtonal tunings
And of course, both should apply to guitars of more than 6 strings.