Currently, the autocompletion of LilyPond command is simple but stupid: for example, when typing
the first suggestions you get are
although \ragged-bottom is used much less often than \relative. See where this is going?
What would be Truly Awesome® (as in Mozilla Firefox >3.0 'Awesome' URL bar) would be to either have the command list in a pre-defined arbitrary (non-alphabetic) order, or, even better, to have an Intelligent Autocompletion Learning Thingy feature, that would automatically put the most used command on top of the suggestions list.
(I suspect the first solution would be a nightmare to maintain, and the second would require significant changes in Sidekick. But still, that would just be Plain Coolness™.)